Technical Report Grammatical error prediction Øistein E. Andersen Number 794

Technical Report Grammatical error prediction Øistein E. Andersen Number 794
Technical Report
UCAM-CL-TR-794
ISSN 1476-2986
Number 794
Computer Laboratory
Grammatical error prediction
Øistein E. Andersen
January 2011
15 JJ Thomson Avenue
Cambridge CB3 0FD
United Kingdom
phone +44 1223 763500
http://www.cl.cam.ac.uk/
c 2011 Øistein E. Andersen
This technical report is based on a dissertation submitted
2010 by the author for the degree of Doctor of Philosophy to
the University of Cambridge, Girton College.
Technical reports published by the University of Cambridge
Computer Laboratory are freely available via the Internet:
http://www.cl.cam.ac.uk/techreports/
ISSN 1476-2986
Summary
In this thesis, we investigate methods for automatic detection, and to some extent correction, of grammatical errors. The evaluation is based on manual error annotation in the
Cambridge Learner Corpus (clc), and automatic or semi-automatic annotation of error
corpora is one possible application, but the methods are also applicable in other settings,
for instance to give learners feedback on their writing or in a proofreading tool used to
prepare texts for publication.
Apart from the clc, we use the British National Corpus (bnc) to get a better model of
correct usage, WordNet for semantic relations, other machine-readable dictionaries for orthography/morphology, and the Robust Accurate Statistical Parsing (rasp) system to parse
both the clc and the bnc and thereby identify syntactic relations within the sentence. An
ancillary outcome of this is a syntactically annotated version of the bnc, which we have
made publicly available.
We present a tool called GenERRate, which can be used to introduce errors into a corpus of correct text, and evaluate to what extent the resulting synthetic error corpus can
complement or replace a real error corpus.
Different methods for detection and correction are investigated, including: sentence-level
binary classification based on machine learning over n-grams of words, n-grams of part-ofspeech tags and grammatical relations; automatic identification of features which are highly
indicative of individual errors; and development of classifiers aimed more specifically at
given error types, for instance concord errors based on syntactic structure and collocation
errors based on co-occurrence statistics from the bnc, using clustering to deal with data
sparseness. We show that such techniques can detect, and sometimes even correct, at least
certain error types as well as or better than human annotators.
We finally present an annotation experiment in which a human annotator corrects and supplements the automatic annotation, which confirms the high detection/correction accuracy
of our system and furthermore shows that such a hybrid set-up gives higher-quality annotation with considerably less time and effort expended compared to fully manual annotation.
Preface
First of all, there are many people who deserve to be mentioned on this page, but whose
names do not appear — from my parents, who hardly ever lost their patience when I
bombarded them with questions as a child, to the visiting friend who presented me with
the idea of a doctorate over dinner at a time when further studies did not seem an obvious
choice —, and I should like to express my gratitude to every single one and assure them
that their various contributions have not been forgotten.
The person most directly involved in the work presented in the following is of course my
supervisor, Ted Briscoe, who has always been available, under whom I have enjoyed considerable freedom, and who has at the same time led me back on track whenever questions
of only tangential relevance to my research topic started to take up too much of my time.
Parts of the work has been done in collaboration with Andreas Vlachos and Jennifer Foster,
and the annotation experiment could not have been successfully carried out without Diane
Nicholls’ kind co-operation. Rebecca Watson and John Carroll have both been very helpful
with technical issues related to the clc and rasp.
Many useful comments have been provided by conference attendees and anonymous reviewers, but also by colleagues in the research group, including my second supervisor Ann
Copestake, and by friends and acquaintances from other fields.
This work would not have been possible without the support from Cambridge esol1 and
the Cambridge Overseas Trust.
Finally, I should like to thank Helen Yannakoudakis for having read through the thesis
in its entirety and contributed to making it more readable than it would otherwise have
been, as well as my examiners, Stephen Pulman and Paula Buttery, for their thorough and
friendly approach and valuable comments and suggestions, many of which have resulted in
improvements to this document.
1
) This dissertation reports on research supported by the University of Cambridge esol Examinations.
Table of contents
1. Introduction
11
1.1. Rules of grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
1.2. Codification of English grammar . . . . . . . . . . . . . . . . . . . . . . .
14
1.3. Grammaticality and acceptability . . . . . . . . . . . . . . . . . . . . . . .
15
1.4. Corporal evidence
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1.5. Automatic error detection . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
2. Right and wrong
23
2.1. Error corpora . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
2.2. Learner corpora . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
2.3. Error classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.3.1. Surface structure taxonomies . . . . . . . . . . . . . . . . . . . . .
27
2.3.2. Linguistic category classification . . . . . . . . . . . . . . . . . . . .
28
2.3.3. Extent and context . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.3.4. Taxonomies used for error annotation . . . . . . . . . . . . . . . .
31
2.3.5. Other approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
2.4. Error detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
2.4.1. Non-word detection . . . . . . . . . . . . . . . . . . . . . . . . . .
34
2.4.2. Isolated word error correction . . . . . . . . . . . . . . . . . . . . .
34
2.4.3. Context-sensitive spelling errors . . . . . . . . . . . . . . . . . . . .
35
2.4.4. Grammar checking . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
2.4.5. Children and foreigners . . . . . . . . . . . . . . . . . . . . . . . .
37
2.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
8
contents
3. Bits and pieces
39
3.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3.2. Annotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
3.3. Error-coded and parsed version of the CLC . . . . . . . . . . . . . . . . . .
44
3.4. Parsing and metadata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
3.5. RASP4UIMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
48
3.6. Processing of the BNC . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
3.6.1. Collection reader . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
3.6.2. Part-of-speech tagging, lemmatisation and parsing . . . . . . . . . .
50
3.6.3. Collection consumer . . . . . . . . . . . . . . . . . . . . . . . . . .
52
3.7. Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
53
3.8. Limitations and further work . . . . . . . . . . . . . . . . . . . . . . . . .
55
4. Binary sentence classification
57
4.1. Machine learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
4.1.1. Naı̈ve Bayes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
58
4.1.2. Balanced winnow . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
4.1.3. Maximum entropy . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
4.1.4. Support vector machines . . . . . . . . . . . . . . . . . . . . . . . .
60
4.2. Sentence selection and feature extraction . . . . . . . . . . . . . . . . . . .
61
4.3. Classification performance and analysis . . . . . . . . . . . . . . . . . . . .
62
4.4. Quantitative effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
4.5. Expert error detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
4.6. Direct comparison of classification results . . . . . . . . . . . . . . . . . .
67
4.6.1. Individual classifiers independently . . . . . . . . . . . . . . . . . .
67
4.6.2. Individual classifiers together . . . . . . . . . . . . . . . . . . . . .
69
4.6.3. Specialised classifiers providing additional features . . . . . . . . .
69
4.7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
contents
5. Synthetic errors
9
71
5.1. Earlier use of artificial error data . . . . . . . . . . . . . . . . . . . . . . .
72
5.2. Error generation tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
5.2.1. Supported error types . . . . . . . . . . . . . . . . . . . . . . . . .
74
5.2.2. Input corpus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
5.2.3. Error generation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
5.3. Classification experiments . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
5.3.1. Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
5.3.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
5.4. Limitations of GenERRate . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
5.4.1. Sophistication of the error specification . . . . . . . . . . . . . . . .
79
5.4.2. Covert errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
5.4.3. More complex errors . . . . . . . . . . . . . . . . . . . . . . . . . .
80
5.5. Conclusion and further work . . . . . . . . . . . . . . . . . . . . . . . . .
80
6. Replacement errors
83
6.1. Adjectival choice errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
6.1.1. Semantic relatedness . . . . . . . . . . . . . . . . . . . . . . . . . .
84
6.1.2. Corpus frequencies vs. annotator judgements . . . . . . . . . . . . .
85
6.1.3. Error detection and correction . . . . . . . . . . . . . . . . . . . . .
86
6.2. Prepositional choice errors . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
6.3. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
88
6.4. Clustering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
6.4.1. Non-parametric Bayesian clustering . . . . . . . . . . . . . . . . . .
91
6.4.2. Evaluation issues . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
6.4.3. Clustering experiments . . . . . . . . . . . . . . . . . . . . . . . . .
94
6.4.4. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
6.5. Experiments using the clusters . . . . . . . . . . . . . . . . . . . . . . . . .
97
6.5.1. Preposition guessing . . . . . . . . . . . . . . . . . . . . . . . . . .
97
6.5.2. Detecting preposition errors . . . . . . . . . . . . . . . . . . . . . .
101
6.6. Discussion and future work . . . . . . . . . . . . . . . . . . . . . . . . . .
104
10
contents
7. Semi-automatic annotation
105
7.1. Status quo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
105
7.2. Automatic pre-annotation . . . . . . . . . . . . . . . . . . . . . . . . . . .
107
7.2.1. Purveyors of perpetual perplexity . . . . . . . . . . . . . . . . . . .
107
7.2.2. Morphological metamorphosis . . . . . . . . . . . . . . . . . . . .
109
7.2.3. Spell-catching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
111
7.2.4. Euphonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
112
7.2.5. Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
112
7.3. Annotation tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
115
7.4. Annotation experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
116
7.4.1. Manual annotation (part 1) . . . . . . . . . . . . . . . . . . . . . .
117
7.4.2. Semi-automatic annotation (part 2) . . . . . . . . . . . . . . . . . .
118
7.4.3. Annotation of individual sentences classified as good/bad (part 3) .
119
7.4.4. Re-evaluation in context (part 4) . . . . . . . . . . . . . . . . . . .
119
7.5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
119
8. Conclusion
121
a. Error taxonomies
123
a.1. Gooficon classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
123
a.2. FreeText error taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . .
124
a.3. SST corpus taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
125
a.4. ICLE/Louvain taxonomy of errors . . . . . . . . . . . . . . . . . . . . . . .
126
b. Encoding systems
127
c. Part-of-speech tags
133
d. XML listings
139
d.1. CLC mark-up from Chapter 3 . . . . . . . . . . . . . . . . . . . . . . . . .
139
d.2. BNC mark-up from Chapter 3 . . . . . . . . . . . . . . . . . . . . . . . . .
143
d.3. From Chapter 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
145
References
155
Chapter 1.
Introduction
he study of grammar can be traced back to ancient India, where it originated as an
auxiliary discipline to the study of the Vedas, the oldest sacred texts of Hinduism.
Amongst the first Sanskrit grammarians were Sakatayana (c. 8th c. bc) and Yaska (c. 7th
c. bc), and the discipline culminated with the completion of Panini’s (c. 5th c. bc) gram8 chapters
mar known as a£[email protected], a description of Sanskrit morphology in 3,959 rules. Prominent
As..tādhyāyı̄
20th -century linguists influenced by Panini’s ideas include Saussure and Chomsky, not to
mention Bloomfield, who went so far as to characterise Panini’s grammar as ‘one of the
greatest monuments of human intelligence’ (1933, p. 11), though its reputation as a concise yet exhaustive description of Sanskrit, whose perfection remains unsurpassed by any
other grammar for any language, may be exaggerated (Mishra 1986). Unfortunately, early
Indian insights into grammar remained unknown in Europe until the 19th century.
T
In the Hellenistic world, the need for linguistic study arose with the development of rhetoric and logic. Concepts like ‘syllables’ and ‘sentences’ were already being discussed in the
5th century bc by the Sophists, one of whom, Protagoras of Abdera, is said to have pointed
out the distinction between the three genders of Greek: masculine, feminine and neuter
(Aristotle Rhet., III.v, p. 1407b ). According to Plato (428/7–348/7 bc), his teacher
Socrates (c. 469–399 bc) observed the distinction between two different kinds of words,
words
names
speech
ῥήµατα and ὀνόµατα, from which λόγοι are made up (Crat., p. 431b ), defined more precisely
in another of Plato’s dialogues as denoting actions and the ones who perform them, respectively (Soph., p. 262a ). Plato’s pupil Aristotle (384–322 bc) notably recognised a
fall
verb not in the present tense or a noun/adjective not in the nominative case as a πτῶσις of
bonds
the primitive ῥῆµα or ὄνοµα (Int., ii–iii, p. 16b ), and also used the term σύνδεσµοι to refer to
words which do not denote actions or actors but whose function it is to bind the sentence
together (Aristotle Rhet., III.v, p. 1407a ).
Grammar started to emerge as a separate discipline with the Stoics (4th c. bc onwards),
who distinguished between a word per se, the mental image it evokes and the ‘thing’ or
joints
‘situation’ to which it refers. At this point, ἄρθρα (articles and pronouns) were separated
12
chapter 1. introduction
from the remaining σύνδεσµοι (conjunctions and prepositions), and the Stoics provided more
parts of speech
accurate and appropriate definitions of the different µέρη λόγου and introduced new distinctions, including the one between active, middle and passive voices. The study of language
continued with the Alexandrians (3rd c. bc onwards), whose grammatical insights were epiArs
Grammatica
tomised in the Tέχνη Γραµµατική (c. 100 bc), attributed to Dionysius of Thrace (Ars Gr.).
This first extant grammar of Greek is mainly a condensed treatise on morphological catnoun
verb
participle
article
pronoun
preposition
egories, defining eight parts of speech (viz, ὄνοµα, ῥῆµα, µετοχή, ἄρθρον, ἀντωνυµία, πρόθεσις,
adverb
conjunction
ἐπίρρηµα and σύνδεσµος) and their accidents (e.g., case, number, person, mood and tense).
Whether Dionysius’ treatment of the subject ‘have been little improved upon for more than
twenty centuries’ (Dinneen 1967, p. 150) or whether ‘the continuity of the classical tradition [ . . . ] often [lie] in the names of the categories rather than in their contents’ (Michael
1970, p. 490) is a matter for another dissertation, as is the question of correct attribution,
exact chronology and relative interdependence of the first Græco-Latin grammars, but it
seems safe to say that the ideas expressed in Dionysius’ τέχνη has had a lasting influence
on the formulation of grammars for European languages until this day: smaller Latin artes
grammaticæ are very similar to it in both form and contents; Apollonius Dyscolus (fl. ad
100) assumed the same categories for his extensive treatises on Greek grammar, including syntax, whereupon Priscian’s (fl. ad 500) ars was based; and subsequent grammarians
largely followed the precedent thus established, also for the study of vernacular languages
rather remote from Greek and Latin morphology and syntax.
One problem with adopting the categories initially discovered in the Greek language, instead of reapplying the methodology used to determine the relevant ones in the first place,
is that the description of one language could easily be influenced by specificities of another:
for instance, Priscian retained the optative mood alongside the subjunctive, despite there
being no morphologically distinct optative verb forms in Latin; and Bullokar’s first English grammar (1586), largely modelled on what is generally known as Lily’s Latin grammar,
kept the case system almost entirely unchanged, only unifying dative and ablative into ‘gainative’, thus characterising English as a language with five cases. Despite shortcomings such
as these, categories first established over two millennia ago are still commonly used, in
particular for the description of European languages.
1.1. Rules of grammar
More relevant to our concerns is the appearance of the concept of grammatical and correct
language. Arguably, any description of a language has the potential to become normative,
in the same way as Panini’s grammar was to become the very definition of classical Sanskrit,
forms and constructions not explicitly mentioned therein thus effectively being outlawed.
speak Greek correctly
The word ἑλληνίζειν is attested in Plato (Prot., p. 328a ); his predecessor Socrates regarded
‘correct language [as] the prerequisite for correct living (including an efficient government)’
1.1. rules of grammar
13
(Guthrie 1971, p. 276); and the preoccupation with proper use of language emerged
even earlier amongst the Greeks. Tέχνη γραµµατικὴ originally signified the modest art of
letters
combining γράµµατα into words (cf. Plato Crat., p. 431e ); by Dionysius’ time, it had
extended to encompass the ‘knowledge of the language generally employed by poets and
writers’ (Ars Gr., I) in its entirety (including, e.g., etymology and literary criticism); but
narrower definitions ultimately prevailed:
Grammatica est scientia recte loq̆ēdi.
— Isodore Orig., I.v, p. 4r
Grammatica eſt recte ſcribendi atque loquendi ars.
— Lily c. 1500
Gramatica. Arte, che’nſegna a correttamente parlare, e ſcriuere.
— Accademia 1612
Grammaire ſ.f. L’art qui enſeigne à parler & à eſcrire correctement.
— Académie 1694
Grammar is the Art of rightly expreſſing our thoughts by Words.
— Lowth 1762, p. 1
Grammar has sometimes been described as the Art of speaking and writing correctly. But people
may possess the Art of correctly using their own language without having any knowledge of
grammar. We define it therefore as the Science which treats of words and their correct use.
— West 1894, p. 35
Grammar [ . . . :] a person’s knowledge and use of a language.
— Hornby 2005
In practice, ‘grammar’ was long synonymous with Latin grammar as laid down by Priscian
et alii, who dealt mainly with morphology and syntax, and is therefore sometimes used
to denote these subdisciplines only, as is indeed the case for many of the grammar books
mentioned in this chapter. Apart from such instances, however, the terms ‘grammar’ and
‘grammaticality’ will in the following assume a more general sense related to linguistic
knowledge and ability as suggested by the quotations above; in other terms, a ‘grammatical’ error may consist not only in a morphologically malformed word or a syntactically
incorrect construction, but also for instance in a non-idiomatic expression or a confusion
between similar words. (Certain low-level errors like accidental spelling mistakes should
perhaps not be regarded as grammatical errors per se, but it is difficult to maintain a sharp
delineation, and their absence would in any case be a prerequisite for grammaticality.)
14
chapter 1. introduction
1.2. Codification of English grammar
The state of the English language started to become a preoccupation in the 16th century,
after the Reformation, as the vernacular was gradually replacing Latin as the language of
learning. Bullokar’s first English grammar presented itself as being
ſu̧ffic᾿i.ent for the ſpe᾿di. lærńi.ng ho̧w t˛oo párc᾿ E᾿ngliſh ſpe᾿ch for the perfecte.r wrýti.ng thær-of, and
uzi.ng of the beſt phráſ̧éζ thær-in,
— 1586
and Cawdrey’s Table Alphabeticall, the first monolingual English dictionary (limited to
‘hard words’, but including English definitions), claimed to
conteyn[ . . . ] and teach[ . . . ] the true vvriting, and underſtanding of hard vſuall Engliſh wordes,
borrowed from the Hebrew, Greeke, Latine, or French, &c.
— 1604
A century and a half later, many more dictionaries and grammar books had been published,
but no norm seemed to gain general acceptance; a writer of English had
neither Grammar nor Dictionary, neither Chart nor Compaſs, to guide [him] through this wide
ſea of Words.
— Warburton 1747, p. xxv, original emphasis.
Italy and France had meanwhile established academies, each of which had produced an
authoritative dictionary and provided definitive advice re correct use of language, whereas
the need for codification of the English tongue was instead to be fulfilled by two seminal
works published just after the middle of the the 18th century: Johnson’s dictionary in
1755, and Lowth’s grammar in 1762. Samuel Johnson compiled, in the space of nine years,
what ‘easily ranks as one of the greatest single achievements of scholarship’ (Bate 1978),
containing over 40,000 entries and 110,000 literary examples, a novelty at the time. He
‘left, in the examples, to every authour his own practice unmoleſted’ (Johnson 1755, p. A4),
and he claimed that he did
not form, but regiſter the language; [ . . . ] not teach men how they ſhould think, but relate how
they ha[d] [t]hitherto expreſſed their thoughts.
— ibid., p. C2
Nevertheless, his approach was clearly not purely descriptive, as illustrated by his ‘adjuſting
the Orthography, which ha[d] been to th[at] time unſettled and fortuitous’ (ibid., p. A3,
original emphasis), evoking etymology and analogy as guiding principles in addition to
established usage. Also Robert Lowth quoted the best writers of his time, but often to
point out that they had ‘fallen into miſtakes, and been guilty of palpable errors in point of
Grammar’ (1762, p. ix), for he considered that
1.3. grammaticality and acceptability
15
pointing out what is wrong [ . . . ] m[ight] perhaps be found [ . . . ] to be [ . . . ] the more uſeful
and effectual manner of inſtruction.
— ibid., p. x–xi
The discipline has seen progress since: a large number of dictionaries and grammar books
have been developed for a variety of purposes; Johnson’s dictionary has been replaced by
the Oxford English Dictionary as the reference; descriptive grammars from around 1900
(including, e.g., Pedersen’s) have provided a more complete description of English; a large
number of different syntactic theories have been developed; modern grammars like Huddleston & Pullum’s have attempted ‘to bridge the large gap [ . . . ] between traditional
grammar and the partial descriptions [ . . . ] proposed by [linguists]’ (2002, p. xv); and the
increasing availability of corpora has made it possible to investigate actual usage quite precisely, thus to some extent obviating the need to rely on intuition and unilateral judgement.
More revealing, perhaps, is the continuity: English orthography has changed very little in
over two hundred and fifty years; rules first formulated by Lowth have now become universal (e.g., the ban on double negatives and the use of were rather than was with you),
though others have since fallen out of favour; the literati rather than οἱ πολλοὶ influence the
language sanctioned by dictionaries and grammar books; and the continued existence of
grammar/style guides like Fowler’s Modern English Usage attests on a sustained quest for
the Holy Grail of ‘propriety and accuracy’ (Lowth 1762, p. ix).
Cawdrey’s dictionary was aimed ‘for the benefit & helpe of Ladies, Gentlewomen, or any
other vnskilfull perſons’ (1604) who had not benefited from a classical education, and Bullokar intended his grammar not only to be useful for the Englishman learning his own and
other languages, but also to be ‘very-aid-fu̧l too the ſtrańg᾿o.r t˛oo lærń e᾿ngliſh perfectl.y and
spe᾿di.l.y’ (1586). The first monolingual English dictionary aimed exclusively at foreigners
did however not appear until 1942, when Hornby’s predecessor of the Oxford Advanced
Learner’s Dictionary was published in Japan; since then, the number of dictionaries and
grammars of this type has multiplied, and Cambridge University Press now asserts that its
pedagogical English Grammar in Use is ‘the world’s best-selling grammar book’.
1.3. Grammaticality and acceptability
Lowth claimed to be the first to use negative examples, in which case he may also have
pioneered the binary division between grammatical and ungrammatical sentences:
The principal deſign of a Grammar [ . . . ] is to teach us to expreſs ourſelves with propriety [ . . . ],
and to be able to judge of every phraſe and form of conſtruction, whether it be right or not. The
plain way of doing this, is to lay down rules, and to illuſtrate them by [positive and negative]
examples.
— 1762, p. x
16
chapter 1. introduction
Chomsky presented an apparently identical goal almost two centuries later:
The fundamental aim in the linguistic analysis of a language L is to separate the grammatical sequences which are the sentences of L from the ungrammatical sequences which are not sentences
of L.
— 1957, p. 13, original emphasis
As pointed out by Sampson, however, traditional grammarians were merely concerned with
the ‘uninteresting’ set of ungrammatical sentences that people are actually seen to produce,
including those which will be labelled as socially deprecated rather than ungrammatical by
agnostic or non-prescriptive linguists, as opposed to Chomsky’s perhaps more ambitious
aim of dividing the universe of combinatorically possible sentences into two (2007). A
different perspective is provided by Burt & Kiparsky in the context of learner errors:
It is common practice for transformational grammarians to prefix any ungrammatical sequence
of words with an asterisk (*). These can include sentences that no one would say. Since we are
only interested in sentences actually spoken or written by people learning [English as a second
language], we will draw the distinction by prefixing spoken ungrammatical sentences [...] with
the dagger (†).
— 1972, p. 2
A similar separation is often made between sentences that are grammatical in the sense that
they are licensed by a formal grammar, and the ones that are acceptable or ‘actually grammatical, i.e., acceptable to a native speaker’ (Chomsky 1957, p. 13), which is useful for
the task of evaluating the extent to which a given grammatical formalism corresponds to a
native speaker’s idea of language. This distinction between grammaticality and acceptability becomes harder to define when the grammar under consideration is the one assumed to
exist within the speaker’s mind. An oft-quoted example of a putatively grammatical albeit
clearly unacceptable construction is a sentence with deep recursive centre-embedding:
(1) The certain reputation the reactions the sentences the grammar the syntactician overwhelmed by clever ideas dreamt up in his dusty study located in the attic generated as
grammatical and his students disparaged as abhorrently complex caused earned him
was to become his only solace in old age.
Whether the rarity, not to say inexistence, of such highly convoluted constructions in practice should be ascribed to rules in the mental grammar or to linguistically more peripheral
factors, such as memory limitations, is of little importance to our concerns. The same
applies to sentences which seem more intuitively syntactically well-formed, but hardly acceptable (at least not outside a somewhat contrived context):
(2) ? Colo[u]rless green ideas sleep furiously.2
2
) ibid., p. 15.
1.3. grammaticality and acceptability
17
Such sentences are sometimes said to lack ‘semantic soundness’, though the conditions
determining whether a given sentence is semantically sound or not, especially in the absence
of context, are at best difficult to lay down.
Apart from the special case of evaluating the adequacy of a grammatical formalism, the
need for a distinction between grammaticality and acceptability seems to be felt most
acutely for constructions which do not occur naturally and will therefore not be employed
in the following.
Ungrammatical sentences obviously include syntactically erroneous sentences:
(3) *Little a boy the ran street up.3
Incorrect constructions of the type often brought about by translation also result in ungrammaticality:
(4) *I am here since two years.
Overly colloquial constructions will typically be considered ungrammatical as well (keeping
in mind that this depends on the context, and that overly formal constructions can of course
be equally inappropriate):
(5) *Him and her don’t want no cake.
Then, there are less clear cases like poetic constructions, the acceptability of which depends
on the context:
(6) ? And all the air a solemn stillness holds.
This leads us to the issue of grammaticality as a gradient:
The degree of grammaticalness is a measure of the remoteness of an utterance from the [ . . . ] set
of perfectly well-formed sentences.
— Chomsky 1961, p. 237
Linguists typically use stigmata made up of asterisks and question marks to indicate different levels of grammaticality; there is no standard system, but the ordering from grammatical
to ungrammatical is usually consistent with Andrews’ classification:
3
) Ex. 3–6 taken from Quirk & Svartvik 1966, p. 10.
18
chapter 1. introduction
p
?
??
?*
*
**
Fullkomlega tæk og eðlileg
Tæk, en kannski svolı́tið óeðlileg
Vafasöm, en kannski tæk
Verri, en ekki alveg ótæk
Algjörlega ótæk
Hryllileg
(Completely acceptable and natural)
(Acceptable, but perhaps somewhat unnatural)
(Doubtful, but perhaps acceptable)
(Worse, but not totally unacceptable)
(Thouroughly unacceptable)
(Horrible)
— 1990, p. 203, original translations
In this thesis, the asterisk (*) indicates a clear instance of ungrammaticality, and the question mark (?), a dubious instance.
Sorace & Keller showed correlation between experimental grammaticality judgements
and the violation of grammatical rules (2005): violation of a ‘hard constraint’ (e.g., agreement or inversion) causes a higher degree of ungrammaticality than violation of a ‘soft
constraint’ (e.g., definiteness or verb class constraints); furthermore, each additional violation decreases the degree of grammaticality, but the violation of several soft constraints
typically results in less severe ungrammaticality than the violation of one hard constraint.
This dichotomous point of view is controversial, but the idea that certain rules are more
important than others seems intuitively plausible and has appeared in many variations. The
potential consequences of gradient phenomena in grammar for syntactic theories and grammatical formalisms are however clearly outside the scope of this thesis (see, e.g., Aarts
2007 and Manning 2003 for more on those aspects). More relevant to our concept of
grammaticality is Lennon’s observation with respect to the following sentence produced
by a German university student of English during a semester in England (shown with a
possible correction):
(7) a. ? There is a dam wall which should protect the village from flood.
b. There is a dam which is meant to protect the village against floods/flooding.
In his opinion, ‘it may be various infelicities occurring in close proximity which persuade
the native speaker he or she has recognized an error’ (1991). On the one hand, this implies
that minor errors in the sense of slightly infelicitous expressions should not be regarded
as erroneous as long as they are few and far between, which seems reasonable given that
neither native nor non-native speakers always choose the most conventional expression or
systematically recognise this kind of mild deviance; on the other hand, and more importantly, not even slight infelicities can be disregarded completely, given that they may add up
to a noticeable error.
We shall return to the issue of grammatical errors, including an operational definition and
different ways in which to classify them, in Chapter 2.
1.4. corporal evidence
19
1.4. Corporal evidence
The idea of regarding works of approved authors as a model worthy of emulation is millennia old and has led grammar writers and dictionary compilers to collect innumerable
examples to serve as ‘reservoir[s] of linguistic usage’ (Kučera 1967), an activity which
reached its pre-digital pinnacle with the millions of quotation slips gathered in a purposebuilt scriptorium and put into pigeonholes by James Murray during the preparation of the
Oxford English Dictionary. Digital corpora can be seen as a natural continuation of this
tradition, furthermore allowing linguistic enquiry to be founded more directly on real language data, as opposed to the intuition and experience of a well-read man supported by
occasional quotations.
The basis [for writing a grammar of English] must be copious materials, made up of continuous
stretches or ‘texts’ taken from the full range of co-existing varieties and strata of educated English,
spoken as well as written, at the present time.
— Quirk 1960
The value of corpora was not immediately appreciated by all schools of linguists, though,
as seen in Robert Lee’s dismissal of Nelson Francis and Henry Kučera’s emerging efforts to
create what was to become the Brown University Standard Corpus of Present-Day Edited
American English, for use with Digital Computers:
That is a complete waste of your time and the government’s money. You are a native speaker of
English; in ten minutes you can produce more illustrations of any point in English grammar than
you will find in many millions of words of random text.
— Francis 1979
Chomsky went so far as to reject naturalistic data as actively misleading:
Any natural corpus will be [wildly] skewed. Some sentences won’t occur because they are obvious, others because they are false, still others because they are impolite.
— 1962
The alternative to corpus data is introspection, relying on a native speaker’s ability not only
to establish whether or not a given sentence is grammatical, but also to pull out relevant
examples from a bottomless reservoir within his own mind, as it were. Though he be able
to do this, the problem of representativity nevertheless remains; the need for external input
may be more obvious to the lexicographer than to the syntactician, but both run the risk
of parochialism:
[Native speakers’] linguistic activity ranges from writing love letters or scientific lectures to speaking upon a public rostrum or in the relaxed atmosphere of a private dinner party. Since native
speakers include lawyers, journalists, gynæcologists, school teachers, engineers, and a host of
20
chapter 1. introduction
other specialists, it follows [ . . . ] that no grammarian can describe adequately the grammatical
and stylistic properties of the whole repertoire from his own unsupplemented resources: ‘introspection’ as the sole guiding star is clearly ruled out.
— Svartvik & Quirk 1980, p. 9
This is not the place for a lengthy discussion of the respective merits and demerits of corporal and noetic sources of linguistic evidence. It seems clear to us, though, that corpora
are crucial to the enterprise of grammatical error detection in at least two ways: a reference
corpus allows us to verify that our idea of grammaticality corresponds to the language produced by those who are considered knowledgeable (e.g., reputable authors), and an error
corpus makes it possible to check that commonly committed errors are handled correctly.
Such collections are not ‘millions of words of random text’; rather, a linguistic corpus is
a collection of texts assumed to be representative of a given language, or other subset of a
language, to be used for linguistic analysis.
— Francis 1979
Issues related to collection, annotation and use of error corpora will be discussed further
in Chapter 2. Questions of corpus encoding and exploitation in general will be treated in
Chapter 3, which includes information on the parsed version of the Cambridge Learner
Corpus and describes the process of parsing the British National Corpus
Finally, whereas corpora are generally stored electronically for convenience and ease of
processing, at least one sizeable printed corpus in the sense of a carefully selected collection of printed matter has existed: Kaeding’s variety of lexical and graphemic frequency
counts was derived manually by a large team of people from a corpus of 11 million words
of text of all genres, ranging from original and translated poetry to military orders and
personal correspondence (1897). The amount of manual labour required obviously made
such projects few and far between.
1.5. Automatic error detection
With the advent of the computer, rudimentary proofreading became an automatable task,
and many people today might be frightened by the idea of having to prepare an important
document without any sort of electronic support or ‘validation’ of their writing. As we shall
see in Chapter 2, however, popular spelling and grammar checkers, useful though they may
be to someone who is aware of their limitations and knows the language well, are still no
substitute for either subeditors or language teachers.
In Chapter 4, we look at the task of detecting ungrammatical sentences as a binary classification problem using a set of correct and incorrect sentences as training data, and the
1.5. automatic error detection
21
performance is compared to a system that aims at detecting individual instances of specific types of error. The development of further specialised classifiers will be described in
Chapters 6 and 7.
A prerequisite for many of the error detection techniques is a sample of correct as well as
incorrect language in an easily exploitable format. Any corpus is finite and thus deficient
in the sense that it does not explicitly exemplify all possible constructions. The idea of
creating synthetic errors will be explored in Chapter 5.
Other partial solutions to this problem can be to supplement an error corpus like the Cambridge Learner Corpus with a larger corpus of correct language like the British National
Corpus, to use encyclopædic or taxonomic resources like WordNet, and to cluster words in
order to explicate latent information from the corpus. Ways to circumvent the data sparsity
problem will be the main topic of Chapter 6.
The problem of insufficiently comprehensive corpora can of course be alleviated, albeit not
fully solved, by developing larger corpora. For an error-annotated corpus in particular, the
amount of manual work required during its preparation is considerable, and partial automation could potentially make the process significantly more efficient. In Chapter 7, we
shall see how a set of error detection techniques combined with a dedicated annotation
tool can make the annotation process more efficient and contribute towards more consistent annotation than completely manual procedures. This constitutes both an application of
error detection and correction techniques and a means by which to obtain larger and better
error corpora which in turn can make those techniques more effective, and thus provides a
natural conclusion to the work presented in this thesis.
Chapter 2.
Right and wrong
n this chapter, the concept of grammatical error will be discussed in more detail, in
terms of what constitutes an error as well as how to classify different types. We shall
discuss two tasks for which a definition of error is paramount: error annotation in corpora
and automatic error detection as a writer’s tool. The two applications are related in other
ways as well: a good writer’s tool should take into account the types of error typically
committed, information which is latent in an error-annotated corpus; conversely, the annotation task can be carried out with the help of an automatic tool, an aspect to which we
shall return in Chapter 7.
I
Corder identified three stages in error analysis: recognition, linguistic description and
psycho-linguistic explanation (1974). Both automatic error detection tools and error annotation schemes tend to provide a description in terms of a category or a suggested correction, whereas neither usually includes a higher-level explanation, partly because the necessary information is unavailable.
Exactly what constitutes an error is a controversial topic, but at the same time crucial for
annotation, identification being a prerequisite. Our concept of error is close to James’s
provisional definition of ‘a language error as an unsuccesful bit of language’ (1998, p. 1).
This includes in particular the subclass of errors (sensu lato) variously termed mistakes,
slips or lapsi calami, namely deviances which the writer would have been able to correct
himself if someone had drawn his attention to them; this is convenient for error annotation
since the writer’s intention cannot normally be established for instances of deviant language
in a corpus, and one of the strengths of automatic error detection tools is exactly that they
can draw the writer’s intention to differences between what he wrote and what he meant
to write.
Lennon defined an error committed by a non-native speaker as
a linguistic form or combination of forms which, in the same context and under similar conditions of production, would, in all likelihood, not be produced by the speakers’ native speaker
counterparts.
— 1991
24
chapter 2. right and wrong
This provides a useful complement to James’s vague definition by pointing out that it would
be inappropriate to compare spontaneous speech to prepared oration or an informal e-mail
to an acclaimed piece of literature. This consideration is of course not limited to nonnative speakers, but rather a consequence of the fact that the language produced by a
native speaker varies with the individual and the situation and does not conform to a single
standard. A similar warning against applying inappropriate criteria of correctness was
expressed by Nicholls (pertaining to a written learner corpus):
[When in doubt,] ask yourself whether a native English speaker of average educational level
might conceivably write the same thing.
— 2007
Corder pointed out that a sentence can be superficially deviant, contextually/situationally
inappropriate or both; moreover, even a well-formed and appropriate utterance may be
erroneous by not conveying the intended meaning (1974). Superficial deviance, which
amongst idealised native speakers only occurs as the result of inattention, can generally be
identified without additional information of any kind; contextual or situational inappropriateness, on the other hand, ensues from lack of knowledge not of the language itself but
of how it is used in a particular context or situation, and this type of error can only be
detected when sufficient information about the context or situation is available, which may
not always be the case for corpus data or text checked by an automatic error detection tool;
finally, utterances that are ‘right by chance’ (ibid.) can only be identified as erroneous if
the speaker’s/writer’s intention can be established to differ from the actual meaning of the
utterance produced, which is usually not feasible in the scenarios considered here. Thus, in
a perfectly annotated written error corpus, all instances of superficial deviance and some instances of contextual inappropriateness will have been identified, whereas no superficially
correct and contextually appropriate utterances will have been found to be erroneous, although, in all likelihood, a certain proportion of them are; the same limitation of available
evidence applies to an automatic error detection tool.
2.1. Error corpora
Already Lowth used authentic examples to illustrate common errors, a practice elevated to
something of an art form by the brothers Fowler, who explicitly chose ‘to pass by all rules
[ . . . ] that are shown by observation to be seldom or never broken’ (1906, p. iii), relying on
contemporary literature and journalism to select ‘blunders’ worthy of mention. In a similar
vein, language teachers at all levels more or less conciously/explicitly take errors committed
by pupils past and present into account when they plan the next lesson, the knowledge thus
accumulated by experienced teachers in turn being crucial for the development of teaching
materials for a given demographic.
2.1. error corpora
25
To our best knowledge, the study of errors committed by professional writers — and, more
generally, native speakers from all walks of life in different writing situations — remains
sporadic and unsystematic; no serious effort seems to have been made to characterise with
any level of precision the types of error typically found in either edited or unedited text
(apart from some studies on keyboard-related typographic errors, e.g., MacNeilage 1964,
as well as smaller-scale collections akin to the one included in Foster 2004), leaving authors of writer’s guides and developers of software tools with no option but to rely on
intuitive and impressionistic approaches. Significant practical and technical hurdles would
have to be overcome to obtain a representative and sufficiently large sample of unedited
text in particular, not to mention the appointment of a team of experts with sufficient
authority to add the appropriate amount of proverbial red ink. On the other hand,
the conviction that a native speaker of a language knows enough about it not only to teach
it to others but to become an expert at designing functional computer aids [. . . has led to] a
considerable oversupply of inferior products,
— Kučera 1992
focusing not on areas proven to be difficult for the intended users, but rather on non-errors
such as ‘the great American bugaboo, [. . . ] the schoolmarm dogma that passives are bad’
(ibid.), a superstition that could easily be overcome by studying a corpus of correct and
successful language.
It has been argued that non-native speakers should be regarded as competent users of their
own, personal idiosyncratic dialect, whereby constructions that sound ungrammatical to
native ears should not be labelled as such, since ‘they are in fact grammatical in terms of
the learner’s language’ (Corder 1971, original italics). Needless to say, moving the goalposts to a different field altogether is utterly unhelpful for someone who wants to improve
his foreign-language skills4 ; it may well be true that a thorough analysis of ‘the learner’s
language’ qua language can be illuminating when subsequently compared to the grammar
of the ‘real’ language, but a reverence for individual differences does not seem wholly justified in this case given that a learner’s grammar will often be internally inconsistent in ways
geographical dialects are not, and more importantly that a learner’s unintentional deviation
from the norm is quite different from a dialect speaker’s adherence to a different norm. A
more valid criticism is that a lop-sided focus on errors obscures what the non-native speaker
gets right as well as more subtle linguistic deviation not normally covered under the concept
of error, or
4
) The idea that ‘[g]rammatical accuracy is not always essential for accurate communication’ (Page 1990) has
affected language pedagogy, and those who merely want to acquire the bare necessities of linguistic competence
in order to make themselves understood might be well advised not to focus too much on errors that are not
crucial for the message to be conveyed effectively. Such considerations are however orthogonal to our concerns;
an error detection tool is aimed at helping authors who actively want to avoid errors, and an annotated error
corpus is useful for someone who would like to investigate the communicative effects of different types of error.
26
chapter 2. right and wrong
that work on the linguistic description of learners’ languages can be seriously hindered or sidetracked by [ . . . ] the ‘comparative fallacy’.
— Bley-Vroman 1983
Partly to avoid this problem, but also since many errors would not be apparent in isolated
words or sentences, non-native error corpora are not limited to erroneous words or sentences, but include entire texts with correct as well as incorrect attributes. The term learner
corpus may therefore be more appropriate, despite the high error rate generally observed.
2.2. Learner corpora
Corpora have been used for language teaching purposes since their inception; vocabulary
frequency information extracted from the Brown corpus was pointed out as potentially
useful already by Francis (1967). Studying the properties of edited text does not provide
much insight into the difficulties faced by those who are still learning the language, though,
and perhaps even less so in the case of foreign learners:
For language teaching [. . . ,] it is not only essential to know what native speakers typically say,
but also what the typical difficulties of the learners of a certain language, or rather of certain
groups of learners of this language, are.
— Nesselhauf 2004
The obvious solution — if, like Allén, we value ‘authenticity’ and find it ‘reasonable to
take a look at real manifestations of language when discussing linguistic problems’ (1992)
— is to use a corpus of language written by learners as the basis of such knowledge.
Akin to ‘normal’ corpora, learner corpora have to be constructed according to strict design
criteria in order to be representative, in this case of a precisely delimited population of
learners. A learner corpus usually contains texts not only in different genres but written
by learners from different backgrounds under different conditions, which means that additional metadata is required; Granger & al. (2007) and Ellis & Barkhuizen (2005, p. 30)
mention variables such as age, sex, mother tongue, social background, task type, topic,
time limitations and use of reference tools. It is thus possible to extract texts written by a
given group of learners or investigate differences between groups, but this fragmentation
also means that the full corpus must be much larger than one aimed at being representative of one type of text produced by one homogeneous group, such as the level of written
business English attained by female French monolingual learners of English at the end of a
Scientific Baccalaureate.
A large number of English learner corpora have appeared during the last few years; Schiftner provided an overview of over two dozen corpora ranging in size from a few thousand
words to tens of millions of words, incorporating texts written by learners from one or
2.3. error classification
27
multiple linguistic backgrounds and including different amounts of annotation, mostly developed at academic institutions (2008, q.v. for essential characteristics and references; see
also Pravec 2002 for further details on the larger/older corpora). The majority of the
corpora contain fewer than one million words, and only three are larger than five million
words: the 30-million-word Hong Kong University of Science and Techonology (hkust)
Corpus and two commercial corpora collected to be used for dictionary compilation and
textbook writing by Longman (10 mill. words) and Cambridge University Press (30 mill.
words). The Cambridge Learner Corpus (clc)5 includes material written during language
examination by learners of English at different levels and from all over the world; half of it
has been manually error-coded and will be used as a realistic source of errors throughout
this thesis.
2.3. Error classification
Perhaps the oldest error categories still in use are the ancient Greek concepts of barbarism,
a word or form corrupted by foreign influence, and solecism, originally a faulty concord.
These terms do not seem to be much in favour by English linguists nowadays, but such
a coarse-grained classification is sometimes useful, as seen in Foster’s definition of grammatical error (sc. solecism) as an erroneous construction made up from individually correct
words (2004), as opposed to misspellings (sc. barbarisms) detected by naı̈ve spelling checkers. For error annotation, more detailed categories are needed if a distinction between
different types of error is to be made at all.
2.3.1. Surface structure taxonomies
Dictionaries of errors are organised alphabetically, at least on a superficial level, although
extensive cross-referencing may allow a more systematic approach than one might at first
suspect. Other similar books are explicitly divided into separate sections corresponding to
different types of error, such as Fitikides’ classification into five major categories (1936):
1. Misused forms (wrong preposition, wrong tense, etc.);
2. Incorrect omission (missing preposition, missing auxiliary, missing morpheme such
as plural -s or past tense -ed, etc.);
3. Unnecessary words (superfluous article, superfluous to, etc.);
4. Misplaced words (e.g., adverbials);
5. Confused words (wrong preposition [sic], wrong noun, etc.).
5
) http://www.cambridge.org/elt/corpus/learner corpus2.htm
28
chapter 2. right and wrong
Similar surface structure taxonomies have been discussed more recently, for instance by
Dulay, Burt & Krashen, who noted that ‘[l]earners may omit necessary items or add
unnecessary ones; they may misform items or misorder them’ (1982, p. 150, original italics), or by James, who preferred a quintipartite division into omissions, overinclusions,
misselections, misorderings and blends (1998, p. 111). Surface structure taxonomies have
been critisised for not taking into account the modern view of language as fundamentally
hierarchical rather than as a concatenation of words like beads on a string, but surface
structure is still used for error annotation, partly because it provides a practical approach,
but it has also been argued that the concept of surface structure is psycholinguistically
sound in the sense that language users often think of errors in such terms (e.g., an error involving the use of a definite plural noun phrase instead of an indefinite one would typically
be conceptualised as a superfluous the).
2.3.2. Linguistic category classification
As an alternative to Fitikides’ surface structure taxonomy, Burt & Kiparsky’s collection
of errors (1972) presents an organisation according to more linguistically motivated major
categories (see Appendix a.1 for more details):
1. Clause skeleton
2. Auxiliary system
3. Passive
4. Temporal conjunctions
5. Sentential complements
6. Psychological predicates
Despite completely different starting points, the two taxonomies actually comprise many
of the same more fine-grained categories and individual types of error (e.g., infinitive for
gerund after preposition, misuse of isn’t it? or missing be from passive construction), the
main difference in substance being perhaps Fitikides’ attention to lexis as compared to
Burt & Kiparsky’s focus on syntax, which is reflected in the way the material has been
organised in each.
A more complete taxonomy of errors in written texts is provided by James (1998, Ch. 5,
q.v. for details), as briefly summarised in the following:
1. Substance errors
a) Mechanical errors
i. Punctuation
2.3. error classification
29
ii. Typographic (keyboard-related)
iii. Dyslexic
iv. Confusibles (anus/onus, cords/chords)
b) Misspellings proper
i. Mispronunciation
ii. Written misencoding (inter-/intralingual)
2. Text errors
a) Lexical errors
i. Formal errors of lexis
α Formal misselection (malapropism, deceptive cognate)
β Misformation (borrowing, coinage, calque)
γ Distortion
ii. Semantic errors in lexis
α Confusion of sense relation (hypo-/hypernym, quasi-synonym)
β Collocational errors, verbosity
b) Morphological errors
i. (noun, verb, adjective, adverb)
c) Syntactic errors
i. Phrase structure
α Misselection or misordering of constituents (modifier, head, qualifier)
ii. Clause
α Combination of phrases
iii. Sentence
α Blends
β Coordination
γ Subordination
iv. Intersentence (cohesion)
α
β
γ
δ
ε
Reference
Substitution
Ellipsis
Conjunctions (over-/underuse, misselection, misplacement)
Lexical
3. Discourse errors
a) Coherence errors
i. Topical
30
chapter 2. right and wrong
ii. Relational
iii. Sequential
b) Pragmatic errors6
i.
ii.
iii.
iv.
Taboos
Size of the imposition
Values
Power and social distance
Extending the traditional division of grammar into morphology and syntax, this taxonomy
systematically and exhaustively lists the different levels at which an error can occur — starting with single letters and punctuation marks, going through words, phrases, sentences and
paragraphs, ending with the semantic content of an entire text — and further characterises
the types of error at each level according to applicable criteria such as part of speech, part
of phrase or origin of the error; the hierarchy of constituent parts is more or less given, but
there are still many ways to subdivide the errors at each level (cf. Politzer & Ramirez
1973 for an example of categories closer to those found in traditional grammars).
2.3.3. Extent and context
In general, local errors such as spelling mistakes are easier to detect and correct reliably
than global ones such as discourse errors (which have, at least partly for this reason and
perhaps regrettably, been somewhat neglected in practical corpus annotation work, but
that is not our concern at this stage). However, as pointed out by Lennon, the degree of
localness depends not only on the ‘extent’ of the error, but also on ‘the breadth of context
[ . . . ] criterial for whether error has occurred’, its ‘domain’, which may vary from a single
morpheme to, in the extreme case, extralinguistic context (1991). In other words, even a
trivial typographic error may require a large amount of contextual information to become
apparent, in which case detecting it may not at all be a simple task despite its minute extent.
There is thus no absolute correlation between the level at which an error belongs in the
linguistic hierarchy and the ease with which it can be handled. For this purpose, a more
appropriate linguistically motivated hierarchy might be based on the level of descriptive
detail needed to detect that something is amiss, originally proposed to explain degrees of
grammaticality (Chomsky 1961): at the lowest level, any sequence of English words is
acceptable; at the next level, words are divided into parts of speech and their combination
6
) Whereas pragmatic considerations, for instance with respect to the right level of formality in a given context,
are indeed relevant in written communication, pragmatic errors are perhaps more common in face-to-face interactions; in particular, the example given for ‘size of the imposition’ is the idea that it will be more acceptable
to ask a random stranger for a cigarette in places where tobacco products are easy to come by and relatively
inexpensive.
2.3. error classification
31
must conform to certain syntactic rules, which would exclude something like the *cater ate
my gymsuit; at a higher level, verbs may be subdivided into ‘pure transitives, those with
inanimate objects, etc.’, nouns subdivided similarly, and the rules refined accordingly, at
which point sufficient information would be available for an anomaly like the *khat ate my
gymsuit to be detected. The practicality of an error classification scheme along these lines,
as well as the adequacy of the resulting categories, remains to be established.
2.3.4. Taxonomies used for error annotation
A problem with extant taxonomies known from language manuals and the linguistic literature, be they organised according to surface structure or based on linguistic categories, is
that it is often not clear to which category a given error should be assigned; for instance,
draftsman for draughtsman could reasonably be classified as a phonetic spelling (written
misencoding), a confusion between homonyms or influence from North-American or Australian spelling conventions. The original author would typically be able to indicate the
cause of an error, but this approach is clearly not feasible for corpus annotation, and there
would in any case remain instances where not even the author is able to tell with certainty,
— or the correct explanation might involve a combination of two or several causes. A more
practical solution is to construct the set of error types in such a way as to eliminate, or at
least minimise the amount of, overlap between categories; one way of achieving this is to
use descriptive rather than explanatory categories.
A related problem appears when a given error can be corrected in more than one way,
such as *friends his for his friends (word order error) or friends of his (missing preposition). In this particular case, having a category for ‘incorrect possessive formation’ would
circumvent the issue, but specific guidelines are typically needed to deal with such cases in
a consistent manner. Another option would be to assign a given error to multiple categories, but this is usually not considered, not least because the annotation task becomes even
harder if one has to ensure that all error categories corresponding to equally good or at
least plausible corrections are included consistently.
Most error taxonomies devised for corpus annotation combine surface structure and linguistic categories, as for instance the scheme used in the clc (see Fig. 2.1): the majority
of the error types indicate both the part of speech involved and the general category of
error, expressed either in terms of word-level surface structure modification (missing word,
unnecessary word, etc.) or in terms of linguistic category (agreement, derivational morphology, etc.); additional error types are used when multiple parts of speech are involved
(e.g., word order) or when the part of speech is found to be irrelevant (e.g., spelling or
register). Errors at the word level and below are categorised along lines similar to James’s
linguistic taxonomy, whereas syntactic errors are dealt with in terms of surface structure.
The error types are largely non-overlapping and self-explanatory, but there are examples of
agv
cn
agn
mc
rc
uc
fq
mq
rq
uq
iq
dq
fa
ma
ra
ua
ia
da
fd
md
rd
ud
di
dd
cq
agq
aga
Punctuation
mt
rt
ut
dt
Determiner
fy
my
ry
uy
iy
dy
Pronoun
(anaphor)
fj
mj
rj
uj
ij
dj
Quantifier
fn
mn
rn
un
in
dn
Conjunction
fv
mv
rv
uv
iv
dv
tv
Preposition
s
sx
sa
cl
l
x
ce
id
as
w
Adverb
Wrong spelling
Spelling confusion
US spelling
Collocation/tautology error
Register error
Negative formation error
Complex error
Idiom error
Argument structure error
Word order
Adjective
ag
m
r
u
Noun
Wrong form used
Something missing
Word or phrase needs replacing
Word or phrase is unnecessary
Word is wrongly inflected
Word is wrongly derived
Verb is in the wrong tense
Countability error
Agreement error
Verb
chapter 2. right and wrong
Unspecified
part of speech
32
mp
rp
up
cd
agd
Figure 2.1. Systematic overview of the error codes in the clc (Nicholls 2007).
somewhat problematic and probably not very useful distinctions such as the one between
determiners and quantifiers, as well as a set of arbitrary rules such as the one saying that
confusion between two words of the same part of speech (e.g., flower and flour) should
be regarded not as an instance of spelling confusables (sx) but instead as a replacement
error (rn in the case of nouns). There used to be a category for deceptive cognates (‘false
friends’), but this one has now been removed since the annotators cannot be expected to
recognise cognates reliably for a large number of different languages; this additional more
explanatory category also overlapped with others, in Scholfield’s words ‘a sure sign of a
faulty scale’ (1995). Assuming that an error can be detected, the annotator should be able
to assign it to a category without knowing the author’s mother tongue or what was going
on in his head when the error was committed; the classification should be based entirely on
what can be observed from the text.
A good review of existing error annotation taxonomies is provided by Díaz-Negrillo
& Fernández-Domínguez (2006). Compared to the clc taxonomy, others used for annotating larger corpora such as the International Corpus of Learner English (icle) at The
Université catholique de Louvain (see Appendix a.4) or the Japanese Standard Speaking
Test (sst) corpus (see Appendix a.3), are mainly organised according to a linguistic classification, including categories for complementation of noun and voice of verb; on the other
2.4. error detection
33
hand, the main difference seems to be the formal organisational hierarchy of error types
rather than the distinctions made between different types of error, and surface structure
categories cannot easily be avoided altogether given that word order errors in particular
are difficult to define in other terms. The FreeText taxonomy (see Appendix a.2) is sometimes characterised as three-tiered, but it would probably be more accurate to say that it
is two-dimensional, combining an error category with a part-of-speech category just like
in the clc, but with two potential improvements, namely additional error categories and
consistent encoding of part of speech.
2.3.5. Other approaches
In addition to the largely descriptive taxonomies described in previous sections, other classifications of errors have been developed with the aim of diagnosing the origin of an error,
explaining it from a language-learning point of view, or evaluating how serious it is, for instance in a communicative perspective. Most of these dimensions fall outside the scope of
this thesis, but it can sometimes be difficult to keep description and explanation completely
separate, as we have already seen in the case of cognates and misspellings which happen to
result in a valid word or an alternative spelling which is acceptable in another geographical
variety of the language. A radical way of avoiding this issue is not to classify errors at all,
but provide only a reconstruction, as suggested by Fitzpatrick & Seegmiller (2004). A
more ambitious approach was chosen by the developers of falko (Fehlerannotiertes Lernerkorpus des Deutschen), who chose to encode description and explanation at different
levels (Lüdeling & al. 2008).
Something that seems to be missing from almost all approaches to error annotation is
a perspective of how it can interact with other means of corpus analysis. For instance,
adding part-of-speech tags is all very well, but this is perhaps something that could be
done automatically, especially if a corrected version of the erroneous passage is provided
anyway; conversely, information latent in the error annotation could be helpful for parsing
syntactically incorrect sentences successfully. More generally, when multiple types or levels
of annotation are to be added to the same text, taking advantage of interdependencies
between them to avoid encoding essentially the same information several times can greatly
reduce the amount of work involved, a principle referred to by Pienemann as ‘economical
exploitation’ of pre-existing annotation (1992).
2.4. Error detection
Precursors to computer-based writing tools emerged in the late 1950s in the form of methods for automatic correction of transmission errors, such as the ones introduced by optical
character recognition, and approximate or ‘elastic’ string matching algorithms, typically
34
chapter 2. right and wrong
needed for personal names that are not always written down correctly; many academic
studies on the general problem of string matching and correction were to follow; and perhaps the first spelling checker to become widely distributed, spell,7 has been around since
1971 (Peterson 1980).
Kukich’s comprehensive literature review (1992) identified three distinct subtasks: nonword error detection (e.g., teh), isolated word error correction (e.g., teh corrected to the)
and context-dependent detection and correction, predominantly exemplified by errors owing to orthographic similarity between two words (e.g., too in the too words detected as
erroneous and corrected to two), but this category also covers other types of inappropriate
use of individually correct words.
2.4.1. Non-word detection
For a rather isolating language like English, with limited derivational and inflectional morphology, non-words can be identified fairly reliably by looking up word forms (sequences
of letters delimited by punctuation marks or white space) in a lexicon. For this to work
well, the lexicon should be of limited size and adapted to the writer: it should ideally be not
only sufficiently comprehensive to include any word used in a text and thus avoid flagging
correct words as misspellings, but also sufficiently limited not to include infrequent words,
technical terms outside the author’s field or variant forms belonging to other dialects given
that the chance of an accidental match between a misspelling of one word and the correct
spelling of another increases with the size of the lexicon. The standard approach is to include a medium-size dictionary of fairly common words and give the user the ability to add
words to a personal word list.
After half a century of existence, non-word detection is often regarded as a solved problem, but there is still room for improvement, for instance when it comes to proper names
or handling of white space (or absence thereof) in noun compounds, not to mention consistency checking in cases where more than one spelling is acceptable (e.g., -ise/-ize verb
suffixes).
2.4.2. Isolated word error correction
Once a non-word has been detected, a system can either simply flag it as misspelt or try to
come up with one or more suggestions as to what the intended word might be. In Damerau’s data, 80 per cent of all misspellings could be explained as resulting from a single
insertion of a letter, removal of a letter, replacement of one letter by another or transposition of two adjacent letters, which are ‘the errors one would expect as a result of misreading,
7
) Now known as International Ispell, unrelated to Unix spell.
2.4. error detection
35
hitting a key twice, or letting the eye move faster than the hand’ (1964). Already the original spell hypothesised corrections based on these transformations (Peterson 1980), an
approach which can be expected to work well for purely mechanical errors, but less so for
cognitive errors unless they happen to have the same characteristics as mechanical errors
(e.g., *decieve). Levenshtein later introduced the concept of edit distance, defined as the
minimum number of atomic operations needed to transform, in our case, a misspelt word
into its correct counterpart; unlike Damerau, he only considered insertions, removals and
replacements, but the resulting Levenshtein distance can easily be generalised to include
transpositions as a fourth category, which gives the Damerau–Levenshtein distance. Other
edit distances have been developed since, and methods have been devised to deal with the
problem of finding a correction in the case of there being no word in the dictionary at
distance 1, and to take into account that certain letters are more often confused, left out or
added, than others (Brill & Moore 2000; Ahmad & Kondrak 2005); in addition, completely different techniques have appeared (see Kukich 1992 for an overview), including
ones based on the use of a similarity key (e.g., Soundex, cf. Russell 1918) or explicit
rules. More recent developments include automatic derivation of rules (Mangu & Brill
1997) and the use of complementary sets of rules to handle errors arising from different
causes (Deorowicz & Ciura 2005), an idea already presented as pre-existing by Peterson
(1980).
2.4.3. Context-sensitive spelling errors
The methods mentioned so far are unable to detect real-word errors, a category found
to constitute between 25 and 40 per cent of the total number of errors in two empirical
studies (Kukich 1992). Classical approaches include hand-written rules, grammar-based
techniques and n-gram statistics (see ibid. and Dickinson 2006). n-grams are usually made
up of either words/lemmata or parts of speech, but the two can of course be combined,
or another variation on the theme can be devised, like Huang & Powers’s reduction of
content words to affixes, combined with complete function words (2001). Lexical bi- or
trigrams can potentially handle local errors like *piece prize for peace prize, but n-grams are
not suitable for modelling slightly less immediate contextual effects such as the likelihood
of desert being a misspelling of dessert in the neighbourhood of crème brûlée and crêpes
Suzette, with perhaps dozens of words intervening, a problem which can be solved by using
surrounding words individually as separate lexical features; the two methods are largely
complementary and can be combined, as suggested by Golding (1995), amongst others.
In a similar vein, Hirst & Budanitsky presented a method for detection and correction
of semantic anomalies (e.g., it is my sincere *hole/hope) by considering local and global
context and looking for orthographically related and contextually plausible alternatives to
a contextually anomalous word. At the level between part-of-speech tags and complete
parses, Sjöbergh suggested to use n-grams of phrase labels (noun phrases, verb chains,
36
chapter 2. right and wrong
prepositional phrases, etc.), potentially combined with actual words/lemmata; rare phrase
sequences would be indicative of an error (2005).
2.4.4. Grammar checking
Context-dependent spelling errors are similar to syntactic and stylistic errors in that more
than a single word must be considered in order for them to be identified, and many of
them are typically handled by ‘grammar checkers’ rather than ‘spelling checkers’. Perhaps
the first widely available tool to deal with syntactic and stylistic issues was the Writer’s
Workbench (Macdonald & al. 1982); during the 1980s, several tools were developed and
commercialised as separate products, and more advanced syntactic checking was gradually
added; in 1992, Microsoft Word and WordPerfect both integrated grammar checkers as
part of the word processor, which has since become ubiquitous and thus made grammar
checking readily available to the general public.
Automatic writing aids have the merit of being able to point out errors that the writer might
not have spotted if left to his own devices, but an author is well advised not to rely on a
grammar checker; currently popular commercial systems have not only low recall, which
means that many common errors will remain undetected, but also limited precision, which
means that correct language will sometimes be flagged as questionable or incorrect, leading
to errors being introduced, induced by the grammar checker, if the user puts too much
trust in the suggestions provided by the machine. The imperfect nature of such tools is not
only problematic for poor spellers and people who find it difficult to express themselves
well in writing; competent writers can fall under the computer’s spell by being lulled into a
false sense of security and actually produce texts containing more errors than they would
without mechanical assistance (Galletta & al. 2005).
It will of course not come as a surprise to a computational linguist that grammar checkers
are imperfect and not on their own sufficient to guarantee correct and good use of language,
and also early users and reviewers seem to have been aware of the limitations:
Just as a paint program won’t make you an artist, a grammar and style checker won’t make you
a professional writer.
— Eglowstein 1991
Exactly how well or how badly different programs perform is difficult to quantify since
the set of errors, and even types of error, they should catch is open-ended. Some studies
have nevertheless been performed, including two focusing on errors committed by university students, Kohut & Gorman using business students’ writing samples (1995) and Kies
using sentences exemplifying the 20 most common usage errors found in a sample of 3,000
essays (2008). The results are not directly comparable given that the two studies use different sets of errors for testing, but the results are similar: the best system in each study
2.4. error detection
37
has an accuracy of 50 per cent or just below, and the other good systems work with an
accuracy of between 30 and 40 per cent. Kohut & Gorman tested software available in
the early/mid 1990s, whereas Kies covered the period 1997–2007 and explicitly mentioned
that no actual progress had taken place during that time. These results are consistent with
Bruce Wampler’s (the original author of Grammatı́k, which became part of WordPerfect)
claim that ‘Microsoft has decided that its [ . . . ] grammar checker is “good enough” and
has stopped significant work on improvement’ (Kies 2008). The allegation that the current/traditional grammar checker in Word should have been somewhat abandoned was
corroborated by Aikawa from Microsoft Research, who explained that the grammar-based
approach to detecting well-known usage errors could not be adapted to other languages
without requiring an amount of work essentially amounting to developing a new grammar
checker from scratch for each language, and that other approaches were therefore currently
under investigation (2008).
Owing to the stagnation of integrated spelling and grammar checkers, separate programs,
now including web-based ones, have seen a certain resurgence lately with for instance
WhiteSmoke8 , Ginger9 and StyleWriter10 .
2.4.5. Children and foreigners
As mentioned in the previous section, grammar/style checkers typically assume that the user
already knows the language well; they were originally developed as an aid for adult, educated, native speakers of English to detect punctuation errors, confusions between similar
words, split infinitives, wordiness, excessive use of the passive voice and other constructions
regarded either as potentially stigmatising errors of grammar or as suboptimal according
to advocates of ‘clear writing’. The types of error more frequently committed by children
(Hashemi, Cooper & Andersson 2003) or by non-native speakers will therefore often
not be covered. Liou, who developed an early system for detecting errors in Taiwanese students’ writing, found that contemporary commercial grammar checkers detected (or misdetected) stylistic errors whilst overlooking significant syntactic errors like *having listening
something for having listened to something (1991). Her system instead used specific rules
to detect common learner errors at the sentence level (e.g., subject–verb disagreement), at
the noun phrase level (e.g., one of followed by singular) and at the verb phrase level (e.g.,
unbalanced coordination like *creates science and lived happily ever after), as well as capitalisation errors. Many of the errors can of course be committed by native speakers as
errors of inattention, but they will not usually be very frequent, and a native speaker is less
likely to be led astray by incorrect diagnoses.
8
) http://www.whitesmoke.com/
9
) http://www.gingersoftware.com/
10
) http://www.stylewriter-usa.com/
38
chapter 2. right and wrong
More recent approaches to error correction developed with learners in mind include parsing with mal-rules (Bender & al. 2004; Foster & Vogel 2004), parsing of different
hypothesised permutations of the original string (Vlugter, Ham & Knott 2006) and statistical methods (Chodorow & Leacock 2000). Several methods will be considered more
in detail in the following chapters.
Generally available tools specifically aimed at non-native writers include ProofWriter11
from Educational Testing Service and esl Assistant12 from Microsoft Research.
2.5. Conclusion
We have discussed the concept of error and proposed as a practical definition linguistic productions that deviate from what one would expect in a given situation, including semantic
mismatch between what was said and what was intended only insofar as this is observable
in the sense of being truly obvious from the situation. Furthermore, errors which are in
principle possible to detect may be obscured, for instance by the lack of contextual or situational information or when sentences are evaluated in isolation and not as part of a larger
unit.
Real linguistic errors have been collected, classified and analysed in different ways for
different purposes; and a series of grammar checkers and other tools for writers have been
developed and become widely used. Unfortunately, though, with the exception of certain
tools aimed at foreign learners, much of the existing software seems to have been developed
without due consideration of the types of error typically committed and therefore useful to
detect.
Given the unavailability of a native-speaker error corpus, the errors used as training and
test data for the work reported in this thesis are all committed by non-native speakers, more
precisely during language examinations and recorded in the Cambridge Learner Corpus. It
would seem plausible that errors committed by adult native speakers should be a subset
of those committed by foreign learners, and somewhat similar to the ones committed by
advanced learners under similar conditions, although this is difficult to verify, and the
proportions of different types of error are likely to be different.
11
) https://proofwriter.ets.org/
12
) http://www.eslassistant.com/
Chapter 3.
Bits and pieces
e have already alluded to the use of corpora as evidence of correct as well as incorrect constructions, which presupposes that the corpus exists in an appropriately
exploitable form. One metaphor for a corpus is a paper tape on which the words are written one after another, but such a representation (in the sense of an electronic equivalent)
is not adequate for many purposes: at the very least, one would probably want to know
where the title of an article ends and the first sentence begins in the absence of punctuation,
even if titles are treated as ordinary sentences; if a corpus contains texts belonging to different genres, or both written and spoken material, the different categories must be separable
somehow; and one might want to add syntactic annotation, develop indexes for efficient
searching, and so on. In this chapter, we look at how the current standard of encoding
corpora in an xml format has emerged, and how information provided by a parser can be
added to a corpus. The importance of parts of speech and grammatical relations thus added for the purposes of grammatical error detection will become apparent in the following
chapters.
W
3.1. Encoding
Almost the entire Brown corpus was ‘competently and cheerfully punched’ by a single operator (Kučera 1967, p. ix), using tried and tested 80-column ibm punched cards with
rectangular holes first introduced in 1928 (Kistermann 1991). The punching scheme for
encoding letters in addition to decimal digits had been in place at least since 1934 (ibid.),
but the character set remained extremely limited: no distinction was made between upper and lower case, and only a dozen punctuation marks were available. This limitation
was overcome by adopting and adapting a notation system devised a few years earlier by
Newman, Swanson & Knowlton for the encoding of us patents (1959). Fundamental
characteristics include the use of a prefixed asterisk to indicate upper case and special
codes for formatting information and document structure (see Appendix b for details). The
40
chapter 3. bits and pieces
encoding of Roman numerals as Arabic numerals surrounded by special codes and the systematic distinction between abbreviation point and full stop (both will appear in sequence
after a sentence-final abbreviation) are examples of neat concepts which have not generally
been imitated by later corpora, whereas the lack of discrimination between ampersand and
plus sign is somewhat surprising. Several versions of the corpus have been produced over
the years (Francis & Kučera 1964), partly to take advantage of technical advances such
as the ability to encode lower-case letters directly, partly to make the corpus more immediately useful for particular applications (e.g., a version without punctuation marks for
word frequency tabulation). The tagged version was encoded in a fixed-width format with
30 characters (or ‘columns’ in punched-card terminology) reserved for the actual word or
punctuation mark and 10 characters for the part-of-speech tag.
The lob Corpus (Johansson, Leech & Goodluck 1978) was encoded using the less restrictive ascii character set from the onset, which reduced the need for frequently used
compound symbols by allowing a larger number of characters to be encoded directly, and
also made it possible to devise a more readable and mnemonic coding scheme. New codes
were added to represent such concepts as the start of a paragraph or a list of items, and
provision was made for European characters including Old English letters, Cyrillic in addition to Greek, and a fairly comprehensive set of accents and other diacritical marks in
addition to the diæresis. Most of the modifications can be characterised either as improvements made possible by a larger underlying character repertoire or as extensions added to
handle characters not typically found in us patents; the fundamental model of a text format
enhanced with a number of specific codes remained. The tagged version was encoded in
two different ways, ‘vertically’ with one word per line akin to the tagged version of the
Brown corpus, and ‘horizontally’ with the text flowing normally, the part-of-speech tag
being added immediately after each word, separated by an underscore.
When the bnc was being designed, there was a desire to record bibliographic information
and other metadata within the corpus itself; previous corpora had to a certain extent documented the material included in a separate reference manual and provided a limited way
of extracting sub-corpora by grouping texts of the same genre together, an approach which
was seen as impractical for the compilers and inadequate for the users of a corpus comprising thousands of texts classified according to media, level, region and other criteria, and not
simply assigned to a monolithic genre. Another major concern was the inclusion of partof-speech tags as an integral part of the main corpus and not ‘only’ in a derived version,
which for instance meant that cannot could not be represented as something like can VM0
not XX0 after tagging since it would then be indistinguishable from can not. Burnard
provides some information on the development of the bnc and its encoding format (1999).
As a solution to these and other issues, and to assure its longevity, it was decided to encode
the corpus in an format based on the Standard Generalized Markup Language (sgml),
with mark-up providing structural information like word and sentence boundaries as well
3.1. encoding
41
as a framework for indicating meta-information. In more technical terms, the encoding
scheme to be used for the bnc, called the Corpus Document Interchange Format (cdif),
was defined as an application of sgml. The more general Text Encoding Initiative (tei)
guidelines were developed in symbiosis with the bnc-specific cdif and have since been
adopted by hundreds of corpora and text collections.
Like earlier corpora, the original version of the bnc was encoded using a limited character
set, this time a subset of ascii,13 but sgml provided a standard means of representing additional letters and symbols through character references, and almost all the ones required
in the bnc had already been standardised and were listed in an appendix to the standard
(Goldfarb 1990).
From the outset, it was envisaged that a large proportion of the written corpus could be
constructed from sources which already existed in electronic form, but the availability of
such texts turned out to be smaller than expected, and the typesetting tapes that did exist
were often too difficult to convert, which meant that much of it had to be scanned or typed
instead. Each method introduces errors, but of different kinds: conversion of electronic
sources may introduce errors if the mapping is not accurately defined, which is particularly
likely to go unnoticed for somewhat rare characters and probably explains the substitution
of Zu\3rich for Zürich; systems for optical character recognition easily confuse visually
similar characters and are likely to deserve the blame for lst instead of 1st and | instead of I;
typists may introduce keyboard-related mistakes such as tele[hone for telephone (given that
[ and p occupy adjacent positions on an English keyboard), but also introduce misspellings
or unconsciously fail to reproduce real or apparent errors in the original, and such errors
are both practically inevitable and difficult to detect (cf. Mitton, Hardcastle & Pedler
2007 for a discussion of errors in the bnc, be they faithfully copied from the original source
or introduced as part of the corpus development).
With the rise of the World Wide Web and the success of its HyperText Markup Language
(html) appeared the idea of ‘an extremely simple dialect of sgml’ (Bray & SperbergMcQueen 1996) which could be processed by much simpler parsers than the ones needed
to deal with the full language. At that point, the twin standards Unicode & iso/iec 10646
defined a universal character set, potentially putting an end to the proliferation of character
sets for different languages and purposes, and this was chosen as the document character
set14 (which is left unspecified by sgml itself) for the simplified dialect. An important guiding principle was that ‘[t]erseness [should be regarded as] of minimal importance’ (ibid.),
which allowed a set of mark-up abbreviation techniques to be eliminated. The resulting
13
) The apparent intention is to restrict the repertoire to iso 646 inv, but this is not enforced by the sgml
header, and a few occurrences of characters outside this set have crept in.
14
) This refers to the ‘abstract’ character set, the repertoire of characters that can occur in a document; a
‘concrete’ realisation of a document can use any ‘external’ character encoding to define a mapping from concrete
bytes to abstract characters; character references always refer to abstract characters irrespective of the external
encoding, which allows characters not covered by the encoding to be used.
42
chapter 3. bits and pieces
Extensible Markup Language (xml) has become much more widespread than its progenitor, and the mark-up scheme used in the third edition of the bnc (Burnard 2007) was
defined as an application of xml. At the same time, named character entities were replaced
by Unicode characters encoded in utf-8, and a number of corrections and improvements
were made at different levels (cf. Prytz 2007).
Burnard estimated that not using sgml tag minimisation in the bnc ‘would have more
than doubled the size of the corpus’ (1999), which appears to be a fairly accurate characterisation of what happened when this facility had to be abandoned with the move to
xml; actually, a superficial examination shows that the corpus size has trebled, but this is
partly due to the addition of lemmatised forms and a coarse part-of-speech category. The
difference in file size can at least partly be offset by more efficient compression achieved for
the more repetitive xml format, though, and the availability of xml tools means that the
corpus is more readily exploitable in this format.
Actual characters instead of character entities generally represent an improvement as well,
given that most tools are now able to handle Unicode, and a couple of meaningless or
unreliable distinctions such as the ones between &half and &frac12 for the fraction ½
or between &ft and &prime for the prime ( ) will not be missed. Some arguably useful
distinctions have also disappeared, however, including the one between a regular hyphen
and one appearing at the end of a line, and thus likely not to be part of the word’s normal
orthography.
In general, the actual conversion from sgml to xml should be fairly straightforward given
adequate tools and sufficient expertise, so it is somewhat disheartening to note that the third
edition of the bnc contains errors not present in the first edition and apparently introduced
as part of the conversion process. Trivial examples include the substitution of < (encoded
as &lt) and > (&gt) for << (&Lt) and >> (&Gt), not to mention truly bizarre cases like
the extraneous semicolon and space introduced into (FR) = R F to yield (FR) = R; F
(the corresponding sgml mark-up being (FR)&prime = R&prime;F&prime). The errors
mentioned here are arguably not of great importance as far as using the corpus for linguistic
enquiry is concerned and quite likely quantitatively less important than the inaccuracies
introduced by the original conversion/scanning/typing, although the very existence of such
unexpected errors somewhat undermines the claim that formats based on sgml or xml are
a solution to the perennial problem of viable long-term data storage. Quantitatively more
important are the thousands of empty elements introduced as a result of the somewhat
unsuccessful conversion of formulæ marked up as <w UNC>&formula in the sgml edition
to <w c5="unc" hw=" " pos="UNC> </w><gap desc="formula"/> in the xml edition, by
which the mark-up defining the formula as a word of unknown part of speech has been
transferred to a spurious space in front of it. These and other errors have been reported
to and acknowledged by Lou Burnard, who has suggested that they will be corrected in a
future version of the corpus to the extent possible without extensive manual intervention.
3.2. annotation
43
3.2. Annotation
In their raw form, corpora may provide the necessary material for lexicon compilation,
different types of empirical investigations of language use such as statistical analysis of word
and letter frequencies, or automatic generation of concordance lists for particular words of
interest to researchers or language workers (cf. Luhn’s keyword-in-context (kwic) index,
1960). A small amount of metadata characterising the individual pieces of text in terms
of their origin, nature and so forth furthermore allows to derive, for instance, information
about lexical variation across genres or depending on the date of publication.
En revanche, someone who would like to study something like the usage of the noun can
would have to go through an overwhelming number of occurrences of the homographic
auxiliary if an unannotated corpus were used as a source of concordances, whereas extracting instances of such a simple concept as a past participle followed by a preposition would
not be possible at all. For a corpus to be useful in scenarios such as these, each word
must be assigned its part of speech. The idea of mechanical part-of-speech assignment had
already been put into practice by the time the Brown corpus was first published (Klein &
Simmons 1963); and such a tool, Taggit (Greene & Rubin 1971), was subsequently put
into use during the development of a part-of-speech-tagged version of the corpus, but the
accuracy was only about 77 per cent (Mitkov 2003), so the necessary post-editing was ‘a
long and arduous process, extending over several years’ (Francis & Kučera 1964) which
was not completed until 1979. In comparison, the Constituent-Likelihood Automatic WordTagging System (claws), used to tag the lob corpus, was able to assign the correct tag for
96–97 per cent of the words (Garside 1987), at least in part thanks to information derived
from the tagged version of the Brown corpus, including tag sequence probabilities used for
disambiguation. For the British National Corpus (bnc), manual post-editing was found
to be impractical; part-of-speech tags were assigned by a newer version of claws with an
accuracy estimated to be over 98.8 per cent (at the expense of leaving almost 4% of the
words with a portmanteau tag, ambiguous between two parts of speech). Only Benjamin
Zephaniah’s poetry was tagged manually, since the tagger was ‘not familiar with Jamaican
Creole’ (Leech & Smith 2000), an extract from which is included below:
Dis poetry is not afraid of going ina book
Still dis poetry need ears fe hear an eyes fe hav a look
Dis poetry is Verbal Riddim, no big words involved
An if I hav a problem de riddim gets it solved,
— bnc file f9m, from Zephaniah 1992
Corpora with manual or manually checked syntactic annotation range from the detailed
and thoroughly checked analyses of 128 thousand words in the Surface and Underlying
Structural Analysis of Naturalistic English (susanne) corpus (Sampson 1995) to the more
skeletal bracketing of c. 3 million words in the Penn Treebank (Marcus, Santorini &
44
chapter 3. bits and pieces
Marcinkiewicz 1993; Marcus & al. 1994), whereas the linguistic annotation in larger
corpora is usually limited to part-of-speech assignment and lemmatisation. Other types
of annotation being largely restricted to specialised corpora, the Manually Annotated SubCorpus (masc) of the American National Corpus, which is currently under development,
aims to fill
the critical need for sharable, reusable annotated resources with rich linguistic annotations [ . . . ]
including WordNet senses and FrameNet frames and frame elements [ . . . as well as] shallow
parses and named entities
— Ide & al. 2008
The examples given so far show that annotation may render a corpus more imminently
exploitable; it represents a valuable ‘enrichment’ (Leech 2005). This view is challenged
by Sinclair, who maintains that ‘[a]nnotation loses information’ by leading the researcher
to take provided parts of speech, for instance, for granted instead of re-examining the
actual words (2004); this particular criticism is related to the idea that ‘a mark-up system
can easily become a prison’ (Scott & Tribble 2006, p. 32): pre-existing corpus annotation,
useful though it may be, reflects a certain way of looking at language, with a limited number
of categories and a given level of granularity, which will not always be appropriate for the
task at hand. To avoid this problem, and for the sake of scalability, Sinclair argues that
‘[a]nalysis should be restricted to what the machine can do without human checking, or
intervention’ (1992), non-automatic annotation of generic corpora being a ‘very flawed
activity’, though this comment does not extend to corpora ‘designed and built for a predetermined application’ (2004).
The remainder of this this chapter is devoted to practical issues of adding annotation to a
corpus.
3.3. Error-coded and parsed version of the CLC
After manual error-coding, an erroneous sentence from the Cambridge Learner Corpus
might end up as the following:
(8) Then <rd>some|a</rd> <sx>though|thought</sx>
<iv>occured|occurred</iv> to me.
The original text as written by the candidate can be extracted, as well as a corrected version
constructed using the corrections provided by the annotator:
(9) a. Then some though occured to me.
b. Then a thought occurred to me.
3.3. error-coded and parsed version of the clc
45
In addition, the error codes tell us that this particular sentence contains three errors: a determiner replacement (misselection) error, an orthographic confusion between two similarly
spelt words, and an incorrect verb inflection.
The error-coding provides explicit information about errors, but is mostly limited to the
lexical level, not in the sense that syntactic errors are ignored, but they are difficult to
analyse automatically since parts of speech and syntactic relations between words are not
indicated. One way of adding syntactic information is to use a parser, both on the original
and on the corrected version of each sentence; the clc has been parsed in this way using
the rasp system (Briscoe, Carroll & Watson 2006), which provides tokenisation, partof-speech tagging and lemmatisation:
(10) a.
rr
dd
rr
vvn
ii
ppio1
.
Then
some
though
occur+ed
to
I+
.
b.
rr
at1
nn1
vvd
ii
ppio1
.
Then
a
thought
occur+ed
to
I+
.
In this example, occured in the incorrect version of the sentence has been mistagged as a
perfect participle (vvn) instead of a preterite (vvd), whereas the spelling error itself has
disappeared through lemmatisation.
Furthermore, the rasp parser provides a parsing tree (see Fig. 3.1) and grammatical relations (see Fig. 3.2). Complete xml code for the example sentence can be found in Appendix d.1.
The rasp system is a domain-independent, robust parsing system for English which is free
for research purposes. It was, in common with other extant publically available parsers,
designed for plain-text input and has only limited ability to handle xml-style mark-up
natively. For the purposes of parsing the clc, Rebecca Watson wrote corpus-specific code
to extract the text to be parsed and construct an xml document incorporating the original
error annotation as well as the added syntactic annotation. A newer version of the parsed
clc has been indexed to make it more useful and usable for linguistic research (Gram &
Buttery 2009).
Issues directly related to parsing the clc will not be discussed further here; however, many
of the issues are the same for different types of corpora, so much of the following discussion
on how best to parse the bnc also applies to the clc and other corpora. As for the choice
of parser, it can be argued that a non-lexicalised parser such as rasp can be expected to
give more reasonable analyses of sentences containing misspellings and other errors, which
is obviously important in the case of a learner corpus, although this has not been tested
experimentally; we chose the same parser for the bnc to minimise the risk of diverging
analyses of the same or related constructions in the two corpora, but several other parsers
are able to produce grammatical relations that may well be sufficiently similar to the ones
provided by rasp for this not to be an issue in practice.
A1
DD
AP
T
S
RR
S
-pct
End
V1
V1
PP
.
P1
.
Figure 3.1. Parse trees grown by rasp from the sentences in ex. 10.
Then
RR
some
though
VVN
occured
II
me
PPIO1
to
Then
RR
a
A1
AT1
thought
NN1
AP
NP
N1
T
S
S
-pct
End
V1
V1
VVD
occurred
PP
.
P1
.
II
me
PPIO1
to
3.4. parsing and metadata
47
ncmod
ncsubj
ncmod
ncmod
iobj
Then some though occured to
rr
dd
rr
vvn
ii
dobj
me .
ppio1 .
det
ncsubj
iobj
Then a thought occurred to
rr at1 nn1
vvd
ii
dobj
me .
ppio1 .
Figure 3.2. Grammatical relations derived from the parse trees for the sentences in Ex. 10.
3.4. Parsing and metadata
As we have seen, the bnc contains word and sentence boundaries as well as part-of-speech
tags (Leech, Garside & Bryant 1994), but no parsing information and thus no facility
to search for or otherwise make use of grammatical relations between words, which have
proven useful in many applications. Various groups of people have parsed the corpus
throughout the years using different tools and approaches. However, most, if not all, have
simply removed all ‘extraneous’ mark-up from the corpus before parsing, which is not
entirely satisfactory since we lose, inter alia, the distinction between titles and running
text, formatting information, named entities and multi-word expressions, not to mention
metadata including genre and provenance of texts and spoken data. (In addition, whitespace modifications for tokenisation purposes will, if employed, cause further divergence
from the original.) It seems to us that the only adequate solution is to keep the original
mark-up intact and add new elements and attributes to indicate parsing information.
As an alternative to corpus-specific xml-handling code as had previously been used for the
clc, the option of integrating the different parts of the rasp system into an existing analysis
framework able to handle xml was chosen, partly because this might reduce the amount of
work required to parse a different corpus.
The Unstructured Information Management Architecture (uima) originated at ibm Research from a need to process initially unstructured data, mainly natural-language documents but also speech and images, with a series of complementary tools in order to produce
structured data readily interpretable by a machine (Ferrucci & Lally 2004); for instance,
the task of extracting a list of company names from a newspaper article might involve a
named-entity recogniser which depends on the output from a part-of-speech tagger which
in turn requires the original text to have been split into sentences and tokens. The lack of
a standard for interoperability between different modules means that substantial effort is
often required to make them work together; a well-defined architecture would solve this
problem by allowing ‘mixing and matching’ of components without worrying about interfacing issues: each part adds new structured information in a predefined way, making it
immediately available as input for the remainder of the processing chain. uima enables different tools to be encapsulated into modules which can, at least in principle, be combined
48
chapter 3. bits and pieces
Parser
Lemmatiser
PoS Tagger
Tokeniser
Sentence Splitter
n
Clause
Dependency
n
MorphObj.lemma
MorphObj.suffix
n
MorphObj.pos
MorphObj.probability
Token
Sentence
Figure 3.3. Analysis engines and annotations added.
freely and take advantage of existing modules for import and export of data. The Apache
Software Foundation provides an open-source implementation15 .
3.5. RASP4UIMA
We16 have made uima interfaces to the five individual components of the rasp system
under the name rasp4uima, the first version of which is publicly available17 . rasp’s sentence splitter, tokeniser, part-of-speech tagger, lemmatiser and parser are hence available
as separate analysis engines to all types of documents which can be handled within the
uima framework. Fig. 3.3 provides a schematic overview of how each module contributes
towards the final result.
Starting from unannotated text, the sentence splitter generates Sentence annotations; the
tokeniser Tokens; the part-of-speech tagger MorphObjs (potentially more than one for
ambiguous tokens) with the features pos and probability, to which the lemmatiser adds
lemma and suffix; and the parser Dependency relations and/or the full parse tree as Clause
annotations. Each module uses annotations typically generated by the previous modules,
but which may alternatively be obtained from elsewhere, like our using sentence boundaries
and tokenisation information already present in the bnc.
We have also made an initial version of a collection reader and consumer (writer) to deal
with the particulars of the bnc; the reader reads a document that adheres to the bnc-xml
format, using the original mark-up to generate sentence and token annotations which can
subsequently be used by the tagger, lemmatiser and parser (see Sect. 3.6 for bnc-specific
details), whereas the consumer constructs a file containing all the original data as well as
information obtained from processing.
As we shall see, multi-word expressions are currently parsed as individual words. rasp
15
) http://uima.apache.org/
16
) Original implementation by J. Nioche; bnc-specific and Unicode-related enhancements by me.
17
) http://www.digitalpebble.com/rasp4uima/
3.6. processing of the bnc
49
handles many such expressions internally, but alternative approaches might assign such
expressions a single part-of-speech tag or propose this as one alternative during parsing
(cf., e.g., Lewin 2007 for experimental analysis of the utility of these different approaches).
The rasp4uima framework is flexible enough to support these alternatives, though we have
not implemented them in the first version.
rasp4uima actually contains a mechanism specifically designed to handle the case of mismatch between tokens and word units: part-of-speech tags are attached not directly to an
atomic token, but instead to a higher-level wordForm which may correspond to any number of tokens. This representation of the linguistic information is inspired by the Morphosyntactic Annotation Framework (maf, cf. Clément & Villemonte de la Clergerie
2005) which deals with morpho-syntactic annotation of specific segments of textual documents. maf is currently under development as an iso standard (iso/dis 24611).
3.6. Processing of the BNC
The bnc contains mark-up identifying sentences and tokens quite accurately, so it seems
reasonable to take advantage of this information already present in the corpus rather than
starting anew, which also alleviates the problem of how to combine pre-existing and additional annotations into an xml file at the end of the processing chain.
(11) <trunc>Any</trunc> anyone who dissolved <mw>more than</mw> ½
<gap desc="formula"/> in rivers/lakes is n’t gon na forget his pilgrimage , y’know .
Ex. 11 shows an example sentence to which we are going to refer throughout this section.
It has been artificially constructed from parts of actual sentences in the bnc to illustrate
specific issues related to tokenisation, truncation, and so on. The full xml representation
can be found in Appendix d.2; only the most essential mark-up is retained in Ex. 11, where
tokens are indicated by separating white space instead of explicit mark-up.
3.6.1. Collection reader
Our bnc-specific collection reader uses a bnc-xml file as input to generate a uima representation of the textual content. Each sentence (s element) in the bnc results in a Sentence
annotation, and words and punctuation marks (w and c elements, respectively) typically
map to Tokens. No exception is made for multi-word expressions, whose constituents are
handled as ordinary tokens. We have occasionally found it necessary to depart from the
tokenisation in the bnc, however, as not doing so would cause obvious problems:
50
chapter 3. bits and pieces
First, a few thousand w and c elements are empty. These spurious elements were removed
prior to parsing and are expected to be removed from a new official edition of the bnc as
well.
Secondly, whereas most contracted forms like isn’t and cannot have been split into two
or several words as appropriate in the bnc, others like let’s and d’you have not, nor have
words separated by a slash like his/her. In such cases, what is marked up as one word in
the bnc has nevertheless been treated as two or more tokens by the parser, as happens for
the sequences rivers/lakes and y’know in the example.
Thirdly, in the bnc-xml, some parts of the original text or transcribed speech have been
removed and replaced by a gap element, for reasons of anonymisation, inaudibility/illegibility, lack of appropriate textual representation, and so forth. In over 55 per cent of the
cases, the redacted text is a name, address or telephone number, whereas tables and figures, illustrations and photographs, foreign material and somewhat complicated formulæ
account for another 35 per cent. Such constituents typically play a syntactic role in the
sentence and should not be ignored altogether; we have tagged all gaps as formulæ (&fo),
which effectively means that they will be handled as noun phrases by the parser.
Finally, parsing spoken data presents specific challenges. The tagger lexicon has been extended to cover interjections and contractions not typically found in written text, but several
speech-specific issues remain unaddressed. One particular problem is related to truncated
words and false starts: sometimes, the speaker stops in the middle of a word, changes his
mind and says something else, often as a replacement for the word he was about to utter
as well as previous ones. Somewhat simplistically, we ignore truncated words (i.e., words
inside trunc elements). This does not always work out quite as nicely as in the example, but
attempting to tag truncated words is unlikely to work well, so this seems like a reasonable
approach given that the mark-up in the corpus does not encode the information required
to reconstruct the complete/corrected utterance with false starts removed or relegated to
parentheticals.
3.6.2. Part-of-speech tagging, lemmatisation and parsing
Once the uima representation has been generated, the tagger, lemmatiser and parser can
be evoked normally, using the generic analysis engines. The current version of rasp, which
includes enhancements originally developed for this project, is fully utf-8 compliant, which
means that tokens can be passed on directly without worrying about non-ascii punctuation
marks, accented letters, and so on. Example 12 shows the textual representation sent to
the tagger.
(12) anyone who dissolved more than ½ [gap] in rivers / lakes is n’t gon na forget his
pilgrimage , y’ know .
xcomp ncmod
ncmod
dobj
[gap]
&fo
in
ii
river+s
nnl2
conj
dobj
/
cc
lake+s
nn2
conj
be+s
vbz
not+
xx
ncmod
aux
gon
vvg
na
to
to
xcomp
forget his
vv0 apps|
det
dobj
ta
pilgrimage ,
nn1
,
y’
ppy
ncsubj
know .
vv0 .
Figure 3.4. The tagger has added part-of-speech tags; the words have been lemmatised; and the parser has added dependency relations (grs). The arrows representing
grs are drawn pointing from head to dependent; the label above the arrow indicates the type of relation, and the one below an optional subtype.
anyone who dissolve+ed more than ½
pn1 pnqs
vvd
dar csn mc
ncsubj
cmod
ncmod
ncsubj
52
chapter 3. bits and pieces
dependent
ta
arg mod det
aux
arg
mod
subj dobj
comp
subj
ncsubj xsubj
obj
csubj
conj
dobj
obj2
ncmod xmod cmod pmod
pcomp
iobj
clausal
xcomp ccomp
Figure 3.5. The gr hierarchy.
The information added to the sentence can be seen in Fig. 3.4. The parsing data is encoded
not as trees, but as grammatical relations (grs) between head and modifier. grs result from
transformation of a derivation tree constructed by the parser. The different relations are
illustrated as a subsumption hierarchy in Fig. 3.5. They capture those aspects of predicate–
argument structure that the system is able to recover and are the most stable and grammarindependent representation available. (Cf. Briscoe 2006 for a more detailed description
of the gr scheme.)
As there is no annotated test data for the bnc, we do not know how accurate the rasp
analyses are. However, as we use an unlexicalised model, we expect performance to be
similar to that on other out-of-training-domain test data (c. 80% gr precision and recall, cf.
Briscoe & Carroll 2006 for details).
3.6.3. Collection consumer
A collection consumer written for the bnc uses the original bnc file as well as the newly
generated annotations to create a new file containing the information from both sources as
illustrated in Appendix d.2.2 which shows how the example sentence would end up.
The words have been numbered (attribute id) and annotated with part-of-speech tags (rpos)
and lemma/suffix (lem/affix). The bnc already contains part-of-speech tags from a slightly
less detailed tagset (c5) and a coarse word-class category (pos) as well as lemmatised forms
derived using slightly different rules (hw); these cannot be used directly for parsing with
rasp, but are kept in the corpus and may be useful for other applications, for instance to
measure tagger agreement.
3.7. distribution
53
3.7. Distribution
We wanted to make the parsed version of the corpus available to others, which involved
two major issues: we could not give the contents to anyone who did not already have
a licence for the bnc itself, and it seemed wasteful to distribute the parsed corpus in its
final form, including all the original data. Our initial plan was to rely on the Oxford Text
Archive for distribution, since they would be in a position to know whether someone had a
bnc licence, but it turned out to be more difficult than anticipated to work out the details
and get an agreement in place. The obvious alternative was to make the parsed version
available ourselves, but we did not feel comfortable with the idea that someone could gain
access to the full bnc from us simply by pretending that they had a licence, and there was
no easy way for us to check whether or not someone were a bona fide user of the bnc. We
then came up with the idea of encrypting the parsed version using the original corpus as
the encryption key; the data distributed by us would thus be useless to anyone not already
in possession of the bnc itself. This method potentially puts the parsed version in the
hands of someone who might have obtained the unparsed version by fraudulent means, but
trying to prevent that seems pointless; our concern is simply that we should not facilitate
unauthorised access to the original data.
The following sentence can be found in the bnc:
<s n="4">
<w c5="AVQ" hw="how" pos="ADV">How </w>
<w c5="VBZ" hw="be" pos="VERB">is </w>
<w c5="NN1" hw="infection" pos="SUBST">infection </w>
<w c5="VVN-VVD" hw="transmit" pos="VERB">transmitted</w>
<c c5="PUN">?</c>
</s>
The parsed version includes the new information, but also everything found in the original:
<s n="4">
<w n="1" c5="AVQ" hw="how" pos="ADV"
rpos="NP1" lem="How">How </w>
<w n="2" c5="VBZ" hw="be" pos="VERB"
rpos="VBZ" lem="be" affix="+s">is </w>
<w n="3" c5="NN1" hw="infection" pos="SUBST"
rpos="NN1" lem="infection">infection </w>
<w n="4" c5="VVN-VVD" hw="transmit" pos="VERB"
rpos="VVN" lem="transmit" affix="+ed">transmitted</w>
<c n="5" c5="PUN" rpos="?" lem="?">?</c>
<grlist parse="1" score="-10.825">
<gr type="ncsubj" head="2" dep="1"/>
<gr type="xcomp" subtype=" " head="2" dep="3"/>
<gr type="ncsubj" subtype="obj" head="4" dep="3"/>
54
chapter 3. bits and pieces
<gr type="xmod" subtype=" " head="3" dep="4"/>
<gr type="passive" head="4"/>
</grlist>
</s>
Since we want to make the parsed version available only to those who already have access
to the unparsed version, there is no need to include the original data, but only the parts
that have been added:
n="1" rpos="NP1" lem="How">How
n="2" rpos="VBZ" lem="be" affix="+s"
n="3" rpos="NN1" lem="infection"
n="4" rpos="VVN" lem="transmit" affix="+ed"
n="5" rpos="?" lem="?"
<grlist parse="1" score="-10.825">
<gr type="ncsubj" head="2" dep="1"/>
<gr type="xcomp" subtype=" " head="2" dep="3"/>
<gr type="ncsubj" subtype="obj" head="4" dep="3"/>
<gr type="xmod" subtype=" " head="3" dep="4"/>
<gr type="passive" head="4"/>
</grlist>
The data can be made more compact by removing xml-style mark-up and instead using as
separators characters which cannot occur verbatim in the data. We used line separator (\n),
quotation mark (") and left angle bracket (<) as follows:
1"NP1"How
2"VBZ"be"+s
3"NN1"infection
4"VVN"transmit"+ed
5"?"?
1"-10.825<2"1"ncsubj<2"3"xcomp" <4"3"ncsubj"obj<3"4"xmod" <4""passive
As an additional space-saving measure, the format was constructed in such a way that optional attributes appear at the end and can often be left out completely when they do not
appear in the corresponding xml (e.g., the affix and subtype attributes in the example).
An even more compact format could of course be devised, but we feel this is a good compromise between compactness and direct correspondence with the original format, which
is crucial for straightforward reconstruction. The data was extracted and converted to this
compact format using a Perl script and xml::Parser, whereas the encryption and, more importantly, the final decryption and merging which has to be effectuated by the end user, are
implemented fairly efficiently in a small C program. The obvious alternative of compressing the encrypted files using a tool like gzip does not work well since the repetitive nature
of xml gets obscured by the encryption, so the compact file format really contributes to
considerably smaller files for distribution than could otherwise have been achieved.
3.8. limitations and further work
55
It was expected that users of the corpus would want to decrypt and merge the entire corpus
and save the complete files to disk. It turns out, however, that it may actually take longer to
read a complete file from disk than to read the two smaller files and perform the decryption
and merging: on our system, a time saving of 20–25 per cent was observed when not using
pre-reconstructed files18 .
3.8. Limitations and further work
One of the major advantages of generic architectures such as uima is that they allow otherwise incompatible processing modules to be combined. In practice, inherent dependencies
limit this somewhat; for instance, rasp’s tagger can only be replaced by one that uses the
exact same set of tags if one wants to use rasp’s parser. Generic frameworks also involve a
certain overhead: when we parsed the bnc, it became apparent that the resources available
(most notably in terms of memory) did not permit us to parse with multiple tags assigned
by the tagger within rasp4uima, despite our being able to do so when running rasp on
a stand-alone basis. uima (and probably other similar sytems) also uses an internal representation which, despite being an application of xml, is not easily exploitable by other
systems, so code has to be written to handle export of data to a different format (by means
of a collection consumer) as well as import of data into the system in the first place (using
a collection reader). Our expectation that a generic framework might almost automatically
provide an interface to different corpora without a substantial amount of work for each
individual corpus was thus not met.
For tasks where free ‘mixing and matching’ of different modules is not a goal, generic
frameworks may be ‘overkill’, and the extra complexity and overhead they introduce may
be better avoided, which in our particular case would be possible by enhancing rasp’s
native xml handling in such a way that it could read the bnc format as input and produce
suitable output directly.
The annotation we have discussed so far has been added to the corpus, mixed with the
original data (‘in-line’),19 a practice regarded by some as an unfortunate historic legacy that
makes it difficult to verify that the original data remain unchanged and often complicated
to retrieve the original, unannotated data from the corpus; the alternative is to leave the
original data unadulterated and instead add the annotation to a separate file or as a separate
layer (‘stand-off’, cf. Thompson & McKelvie 1997), which is now common practice (see
Przepiórkowski & Bański 2009 for an overview of of some of the major frameworks).
Various representation standards and annotation tools have emerged over the past decade and
have contributed to some convergence in practice, but at the same time, there has been growing
18
19
) This result was obtained by outputting c. 1/10 of the files in the parsed bnc to /dev/null or to the terminal.
) The format developed for distribution of our parsed version of the bnc keeps the added data separate from
the original, but only until the two are merged.
56
chapter 3. bits and pieces
recognition that interoperability among formats and tools, rather than universal use of a single
representation format, is more suited to the needs of the community and language technology
research in general.
— Ide & Suderman 2009
This has led to the development of ‘pivot’ formats such as the Graph Annotation Framework (GrAF; Ide & Suderman 2007), intended to facilitate interoperability by providing a
common representation.
Another advantage of stand-off annotation is that it automatically allows concurrent hierarchies (e.g., the division of a book into into pages and columns on the one hand and into
sentences and words on the other) even if the mark-up language imposes a tree structure
(as does xml), simply by putting each hierarchy in a different file or on a different layer.
This does however not solve the less common problem of truly non-hierarchical structures,
a problem which can only be solved by using a more powerful mark-up language (DeRose
2004; Di Iorio, Peroni & Vitali 2009).
To conclude this chapter, a few remarks on the xml format might not be out of place. A
welcome uniformity at the mark-up level has been achieved thanks to sgml and xml (see
Appendix b for comparison with pre-sgml formats), but there are still many options for
the higher levels, and different annotation guidelines differ in their recommendations, so
it is not the case in general that an xml corpus will be immediately comprehensible by a
wide range of tools. For certain applications, a non-xml format may be appropriate, and
the need for a widely understood format for data exchange should not unduly influence the
choice of formats for internal representation; xml is to be regarded as an option amongst
others, not as a panacea. We saw that our simple text-based compact distribution format
not only resulted in smaller files, but also, somewhat unexpectedly, could be used to reconstruct the full xml files in less time (including decryption) than it would take to read them
from disk. Another solution to xml’s inherent inefficiency is to use an alternative serialisation devised for increased processing efficiency and compactness, such as the Efficient xml
Interchange (exi) Format (Schneider & Kamiya 2009), which can be seen as introducing
the efficiency of a task-specific binary format into the xml world.
Chapter 4.
Binary sentence
classification
s we saw in chapter 2, commercial grammar checkers typically rely on hand-crafted
rules to detect a restricted set of errors. Atwell described an early attempt to avoid
this (1987), and others have trained classifiers on artificial errors. Foster argued that genuine samples are needed (2004), but the idea of training a classifier using real-life examples
of incorrect constructions was merely suggested, as it would require a much larger corpus
than the one she had compiled.
A
The clc provides a large quantity of English text with a high error rate, and the error
annotation identifies the errors and gives suitable corrections, which makes a supervised
machine learning approach feasible.
Reliable identification, classification and correction of each individual error in a text is a
difficult task; not even a human expert is able to provide reliable corrections for errors
which obscure the author’s intended meaning, and even determining whether a piece of
text is correct or not may require detailed knowledge of the subject matter. Verification
of individual words out of context, on the other hand, would amount to spelling rather
than grammar checking. Classification of individual sentences as being either correct or
incorrect is situated somewhere between the two in difficulty and can be approached with
machine learning techniques, using features extracted from correct and incorrect sentences
as training data for supervised learning.
4.1. Machine learning
In general terms, a supervised classifier is trained using a set of couples (f, c) consisting
of a feature vector f of features fi associated with a class c; subsequently, the classifier
58
chapter 4. binary sentence classification
should be able to assign an unseen vector f to the class c to which it belongs. The couples
(f, c) can be said to constitute the experience from which the machine has been able to
‘learn’ appropriate generalisations, whence the term ‘machine learning’. For the binary
sentence classification task, there are two classes, correct and incorrect sentences, and each
feature vector is intended to give a salient representation of the corresponding sentence,
typically encoding characteristics such as the individual words from which it is made up,
combinations of adjacent or grammatically related words, parts of speech and sentence
length.
The selected machine learning algorithms are the ones previously shown to perform well
on this task (Andersen 2006). Compared to previous work, the results reported in the
following are obtained using cleaner data (in particular, excluding erroneous sentences for
which no correction is given, which were previously treated as effectively being both correct and incorrect), and all the classifiers have been trained on the same amount of data.
The Machine Learning for Language Toolkit (mallet)20 provided implementations of all
the machine learning algorithms used, apart from the support vector machine which was
instead provided by the svm-perf21 implementation (Joachims 2006).
4.1.1. Naı̈ve Bayes
According to Bayes’ rule, the probability that a sentence described by a feature vector f
belong to a given class c, P(c|f), can be reformulated as follows:
P(c|f) =
P(f|c) P(c)
P(f)
P(f) is constant for a given sentence and need not be evaluated if we are only interested in
finding the most likely c,
ĉ = argmax
P(f|c) P(c)
c
P(f)
= argmax P(f|c) P(c).
c
The conditional probability P(f|c) is unfortunately difficult to estimate directly from training data, due to data sparsity (unique f for most distinct sentences unless the feature set
is extremely restricted); the standard solution is to assume that all the features fi in f are
independent, which allows us to express the most likely c as
Y
ĉ = argmax P(f|c) P(c) = argmax P(c)
P(fi |c).
c
c
i
It would of course be the pinnacle of naı̈veté to believe that the words in a meaningful sentence are completely unrelated, but the strong independence assumption underlying naı̈ve
Bayesian classification does not actually hamper performance at all to the extent one might
have expected.
20
) http://mallet.cs.umass.edu/
21
) http://svmlight.joachims.org/svm perf.html
4.1. machine learning
59
4.1.2. Balanced winnow
Littlestone introduced a new machine learning algorithm (1988), called winnow by analogy with the agricultural method used for separating grain from chaff because it is ‘designed for efficiency in separating relevant from irrelevant attributes’ and thus learn from
data with a portentially large proportion of irrelevant attributes, a task that is particularly
important in high-dimensional feature spaces such as the ones that appear when words and
n-grams of words are used as features.
The balanced winnow algorithm is parameterised by an update parameter ε and a near-miss
threshold δ and maintains, for each class c, a weight vector wc with the same number of
dimensions as the number of features and initialised to (1, 1, . . . , 1). The classifier predicts
the class
X
ĉ = argmax wc · f = argmax
wc,i fi .
c
c
i
During training, if the predicted class ĉ is equal to the actual class c, the classifier has
not made an error, and nothing needs to be done; if, on the other hand, the prediction is
incorrect, ĉ = c )= c (or, at least in the mallet implementation, if the classifier almost
chose the second best class c )= c instead, as defined by the inequality wc · f − wc · f =
(wc − wc ) · f < δ), the weight vectors wc and wc need to be updated in order to reflect the
training data more accurately at the next step k + 1:
∀i,
w(k+1)
= (1 + ε)fi w(k)
c,i
c,i
and
w(k+1)
= (1 − ε)fi w(k)
c,i
c,i .
(Given (1 ± ε)0 = 1, only the weights corresponding to non-zero features fi )= 0 in the
misclassified instance will be updated.) The original formulation used (1 + ε)−fi instead of
(1 − ε)fi , which makes the update procedure symmetrical. The otherwise similar Perceptron
algorithm differs by doing additive updates instead of multiplicative ones.
4.1.3. Maximum entropy
The principle of maximum entropy, as formulated by Jaynes, says that
in making inferences on the basis of partial information we must use that probability distribution
which has maximum entropy subject to whatever is known. This is the only unbiased assignment
we can make; to use any other would amount to arbitrary assumption of information which by
hypothesis we do not have.
— 1957
A classifier based on this principle is trained in such a way as to maximize the entropy
H(p) = −
X
f,c
p(f, c) log p(f, c)
60
chapter 4. binary sentence classification
whilst remaining consistent with the partial evidence available in the form of training data.
In the case of no training data available, this will give a uniform distribution. Given feature
vectors with binary features fi , the constraints imposed by the training data can be expressed
by the set of equations
∀i, Ep [fi ] = Ep̃ [fi ],
where
Ep [fi ] =
X
p(f, c) fi
f,c
is the expectation of fi predicted by the model, and
X
Ep̃ [fi ] =
p̃(f, c) fi
f,c
is the empirical expectation of fi evaluated on the training data. According to the principle
of maximum entropy, the best model amongst the ones that satisfy the constraints,
P = p | ∀i, Ep [fi ] = Ep̃ [fi ] ,
is the one of maximum entropy, namely
p∗ = argmax H(p).
p∈P
4.1.4. Support vector machines
Support vector machines (svms) are based on the idea of a hyperplane H separating the data
into two classes (assuming that the classes are actually linearly separable). A hyperplane
can in general be identified by a normalised perpendicular vector n and the distance d from
the origin; the hyperplane contains any point x satisfying
n · x − d = 0.
Given a set of training vectors (fi , ci ) with ci ∈ {−1, +1}, the condition that the hyperplane
must separate the classes can be expressed as
∀i, ci n · fi − d > 0.
In order to maximise the chances that unseen instances will fall on the appropriate side
of the hyperplane, we want to maximise the distance between the training vectors and the
hyperplane. To achieve this, we may introduce two auxiliary hyperplanes H+ and H− ,
parallel to H and shifted by ±δ, and require that they satisfy the same condition as H,
which gives the expression
(
ci n · fi − (d + δ) > 0 ;
∀i,
ci n · fi − (d − δ) > 0
4.2. sentence selection and feature extraction
61
or, equivalently,
∀i, ci n · fi − d > δ.
The sought hyperplane is obtained by maximising δ, and the vectors that limit δ and thus
prevent H+ and H− from moving further apart are called support vectors.
An important generalisation is the substitution of the scalar product by a kernel function,
which essentially transforms the problem into a higher-dimensional space in which the
vectors will be linearly separable if an appropriate kernel function is chosen. Another
extension is the use of a soft margin, which in particular makes the method more robust
with respect to misclassified training instances which would otherwise prevent a separating
hyperplane from being found, formalised by introducing slack factors ξi , which gives
∀i, ci n · fi − d > δ − ξi .
We still want to maximise δ, but at the same time minimise the accumulated degree of
P
misclassification ξi .
i
Cortes & Vapnik provide a detailed description of svm techniques (1995).
4.2. Sentence selection and feature extraction
Data for training and testing was obtained from the parsed version of the clc (cf. Sect. 3.3).
In order for the classifier to be able to learn significant differences between correct and incorrect sentences, as opposed to coincidental correlation between vocabulary and correctness
that might exist in the training data simply because certain words happened not to occur in
both correct and incorrect sentences, only sentences for which both a correct and an incorrect version exist were selected. The resulting training and test sets were therefore balanced
in the sense that they contained the same number of correct and incorrect sentences. The
selection procedure excluded both sentences without errors and those for which no correction was provided, as well as sentences that had been split or merged, typically because a
comma had been changed into a full stop or vice versa.
Fig. 4.1 gives a summary of the features found to be useful for this sentence classification
task: morphologically analysed words and part-of-speech tags used in the n-gram features
are taken from the parse tree, combined with adjacent ones as necessary to form bigrams
and trigrams; unlabelled grammatical relations are simply extracted from the parser output;
the binary feature indicating a parsing error is set if there is no complete parse for the
sentence; and the fragment features indicate whether the parse was created using a fragment
rule and, in that case, the type of its constitutive fragments.
The possibilites for extraction of features from the parsed sentences are by no means exhausted; some obvious extensions include features combining words and part-of-speech
62
chapter 4. binary sentence classification
Feature
Examples
Word
Word bigram
Part of speech
Part-of-speech bigram
Part-of-speech trigram
Grammatic relation (words)
Grammatic relation (parts of speech)
a; thought; occur+ed
a thought; thought occur+ed
at1; nn1; vvd
at1 nn1; nn1 vvd
at1 nn1 vvd
thought→a; occur+ed→thought
nn1→at1; vvd→nn1
Parsing error (binary)
Sentence fragments (binary)
Sentence fragments (clause type)
parsing error
fragment
np fragment; pp fragment
Figure 4.1. Overview of the features used. The examples in the first part of the table are taken from the
fragment aat1 thoughtnn1 occurredvvd .
Classifier
Naı̈ve Bayes
Balanced winnow
Maximum entropy
Support vector machine
Training data
Test data
80.36%
95.25%
79.41%
73.65%
70.39%
69.76%
71.27%
70.88%
Figure 4.2. Accuracy (i.e., the proportion of sentences correctly identified) for different classifiers, all trained
on 346,112 sentence pairs, each comprising a correct and an incorrect version of the same sentence, and tested
on 43,352 pairs. All differences in test accuracy are statistically significant, thanks to the large number of test
sentences. The size of the svm model was restricted by available memory, which is likely to have limited its
performance. This issue was not investigated in any detail, however, since thorough comparison of different
machine learning techniques was not the primary focus of the research.
tags to form ‘mixed’ n-grams, as well as ones encoding the type of grammatical relation
existing between a pair of words or part-of-speech tags.
4.3. Classification performance and analysis
Several classifiers were trained and tested using the data described in the previous section.
Fig. 4.2 shows the accuracy obtained for different machine learning algorithms on the task
of classifying sentences as being either correct or incorrect. There is a lot of variation in
performance on the training set, but the classifier that does best on the training set is the one
that does worst on the test set, so it seems to have achieved this by learning idiosyncrasies
rather than general characteristics. All the classifiers achieve an accuracy of around 70 per
cent on the test set; the maximum entropy classifier gets the best result with 71.27 per cent
accuracy, although it is possible that the svm classifier could have done as well or better if
it were not for memory limitations.
The accuracy is highly dependent upon the type of error, as indicated in Fig. 4.3. Spelling
mistakes obtain the highest detection rate, closely followed by inflectional and derivational
errors, all of which tend to result in intrinsically malformed words. Verbal tense errors are
4.3. classification performance and analysis
63
59%
30,000
62%
20,000
60% 79%
10,000
68% 58% 69% 76%
61% 77%
0
or
or
or
rd
rd
on
rm
ror
ror
nse
err
wo y wo g err g fo ct te nt er n er er err flecti
nt sing
n
r
o
n
e
e
i
n
e
i
d
o
i
t
r
rr
sa pell
Wr Inco greem eriva rd o rrect
cem Mis eces
S
o
n
pla
o
D
A
n
e
c
W In
U
R
Figure 4.3. The hatched bars represent the total number of errors of each category in the test set, and the
open bars represent the number of errors occurring in sentences identified as incorrect by the classifier. The
percentage shows the proportion of errors that occur within sentences classified as erroneous. It might be
worth pointing out that the classifier cannot in general be assumed to have identified each individual error
explicitly: in a sentence with multiple errors, each of them may result in characteristic features which only
together enable the classifier to detect that the sentence is incorrect; moreover, an error that does not engender
any discernible features will nevertheless seem to have been detected if the sentence in which it appears has
been identified as incorrect as the result of another error. (Data from the naı̈ve Bayes classifier.)
20,000
49%
60%
10,000
0
65%
1
2
3
69%
4
72% 76% 83%
5
6
7–
Figure 4.4. The hatched bars indicate the total number of sentences with a given number of errors in our
test set, and the open bars indicate the ones that are correctly classified as erroneous. The percentage is the
proportion of incorrect sentences correctly identified as such. (Data from the naı̈ve Bayes classifier.)
located at the opposite end of the spectrum, and the system hardly does better on errors
involving replacement, insertion or deletion of words; these errors are clearly harder to
spot, not only because the mistake consists in a prohibited combination of individually
correct words, but also because the wrong sentence may be grammatically correct, merely
conveying a message different from the one the writer intended.
Accuracy as a function of the total number of errors per sentence is shown in Fig. 4.4,
which clearly indicates that a higher number of errors in a sentence makes it more likely
to be caught by the system. This is hardly surprising, given that a sentence with multiple
errors will contain many discernible features, whereas a sentence with one single, possibly
minor, mistake will have much in common with a perfectly correct sentence. Conversely,
a correct sentence does not differ substantially from a slightly incorrect one and may thus
mistakenly be classified as incorrect.
The strong effect on performance caused by the mere quantity of errors in a given sentence
suggests that we may want to look at potential errors individually, which would also make
64
chapter 4. binary sentence classification
Feature
Examples
Article + letter
Article + word
a g; an u; a u; an i
a green; an unusual; a useful; an iron
Figure 4.5. Overview of the features used. The examples are taken from the phrases a green man, an unusual
task, a useful tool, and an iron, all of which are correct.
it possible to use more directed approaches to various kinds of errors. An entire sentence
would then be classified as incorrect if at least one error was identified with a high degree
of confidence.
4.4. Quantitative effects
In the sentence-classification experiment described, there are many sources of noise and
several things going on at the same time, which makes it difficult to identify the different
contributions and draw clear conclusions. We therefore wanted to explore the influence
of the number of errors per sentence on classification performance in a more controlled
setting in order to check whether the quantitative effect remains when the naı̈ve Bayesian
classifier is given as input a much more restricted set of features aimed at detecting a single
type of error.
For this experiment, we chose to look at the relatively simple problem concerning the form
of the indefinite article, which shall be either a or an depending on whether the following sound is consonantal or vocalic. There is a certain discrepancy between spelling and
pronunciation (consider, e.g., an hour, an MP and a European), so the correct form is not
immediately obvious from the following letter. We therefore extracted two distinct features
for each occurrence of the indefinite article: 1) the article combined with the following
letter, and 2) the article combined with the following word (see Fig. 4.5). Thanks to the
clc, both correct and incorrect examples could be extracted and used for training.
The initial result on our test set is indicated in Fig. 4.6 and corresponds to 55 per cent
recall of incorrect sentences with 80 per cent precision, which is hardly impressive on such
a simple task. When we looked more closely at the data, however, it turned out that many
of the overlooked errors occurred within sentences with multiple instances of the indefinite
article; in fact, 71 per cent of the incorrect sentences that contain one or more additional,
correct, instances of the indefinite article are incorrectly classified as correct, as are 82 per
cent of the sentences with two or more such instances. A possible explanation could then
be that the evidence for an error was simply drowning in evidence for the contrary, making
the sentence ‘predominantly correct’ in the probabilistic eyes of the classifier.
We then ran a new experiment, in which not entire sentences, but single occurrences of the
indefinite article were to be classified as correct or incorrect. As indicated in Fig. 4.7, 95
per cent of the incorrect occurrences were found, and very few correct occurrences were
4.5. expert error detectors
Classification
Correct (p > 0.5)
Incorrect (p > 0.5)
65
Correct
Incorrect
162,394
51
166
199
Figure 4.6. Sentence-level classification. Results shown as number of sentences. The correct sentences include
those that do not feature the indefinite article, but they are all classified as correct by the classifier, so this does
not affect precision and recall of incorrect sentences.
Classification
Correct
Incorrect
Correct (p > 0.5)
Incorrect (p > 0.5)
26,947
34
17
314
Figure 4.7. Classification of each occurrence of the indefinite article.
mislabelled (90% precision). This seems to indicate that individual examination of potential
errors may indeed be fruitful.
A closer inspection of the classifier output shows that the classifier correctly classified the
vast majority of the data with high confidence (p ≥ 0.9 for the chosen class label), as
indicated in Fig. 4.8; it missed only two errors (*a HTV and *a MC) and apparently gave
ten false positives with this high degree of confidence. However, four of the ‘false’ positives
turn out to be real errors overlooked by the annotators, one is due to a transcription error
(can transcribed as c an with a superfluous space), and the remaining are occurrences of
the letter a rather than the indefinite article a, a problem that could have been avoided
using part-of-speech tagging. The quasi-totality of the misclassifications are thus done with
a lesser degree of confidence and mainly concern somewhat irregular or difficult cases like
underground, universal, US and historic; these can be checked manually, or the classifier
can be improved, be it by means of more training data or by using more salient features.
4.5. Expert error detectors
After having demonstrated that concentrating on single errors could indeed be beneficial,
we wanted to attack a slightly more complex error type, requiring some knowledge of grammar as opposed to mere juxtaposition of words. In this section, we look at a deterministic
system for error detection, with no machine learning involved. Agreement between determiner and noun was chosen as a well-defined, purely grammatical, sentence-internal problem
Classification
Correct
Incorrect
Correct (p ≥ 0.9)
Correct (p < 0.9)
Incorrect (p < 0.9)
Incorrect (p ≥ 0.9)
26,872
75
24
10
2
15
22
292
Figure 4.8. The results from Fig. 4.7 broken down by the probability for the chosen class as calculated by the
classifier.
66
chapter 4. binary sentence classification
Relation involving this
Correct
Incorrect
No link to plural noun
Link to plural noun
48,590
326
326
1,105
Figure 4.9. Presence or absence of a link between this and a plural noun in correct and incorrect instances of
the singular determiner.
that does not rely on the meaning of the words involved or other less clear-cut concepts,
and we then focused on the misuse of this when these is needed, by far the most common
confusion of this kind in the clc.
(13) a. *this/these friends
b. *this/these old school friends
(14) a. This is what good friends do.
b. I need this/these for a meeting with good friends of hers.
Ex. 13 shows that we really need to identify the noun determined by this/these and examine
its number in order to be able to choose between the singular and the plural determiner.
Moreover, this/these does not always determine a noun at all, as illustrated by Ex. 14, and
in this case no determiner-noun agreement should be attempted.
The rasp parser is able to identify the grammatical relation between friends and these in
both phrases in Ex. 13, and it correctly refrains from establishing a direct link between
this/these and any of the nouns in Ex. 14. Because the rasp system is aware of agreement
rules, no connection will be made between the plural noun friends and the incorrect singular
determiner this in Ex. 13, but this is, alas, not a reliable indication of error,22 given that
this/these does not always determine a noun. A possible solution is to make rasp ignore
the requirement of number agreement between noun and determiner; then, the system will
allow a singular determiner like this to act on a plural noun like friends, and the presence
of a grammatical relation between a singular determiner and a plural noun (or vice versa)
should be a good error indication.
We parsed the sentences containing this with this slight modification to the parser and found
that the impossible relation between singular determiner and plural noun was established
in 77 per cent of the sentences containing an incorrect instance of this, and in only a
very low proportion of the full set of correct sentences (see Fig. 4.9). Quite a few of the
incorrect instances were actually impossible to spot due to interactions with other errors
22
) If, on the other hand, no grammatical relation contains this, then something is probably amiss. Only a small
portion (at most 20% in our experiments) of the incorrect sentences can be found by exploiting this directly,
though, for the parser will often find another (incorrect) grammatical relation involving this.
4.6. direct comparison of classification results
67
in the text, as for instance this job corrected to these jobs; we therefore re-evaluated the
system’s performance on the set of incorrect sentences in which the determiner agreement
error appears in isolation, which gave a recall of 92 per cent or 197 out of 215 incorrect
uses of this correctly identified as such. The remaining 8 per cent were missed partly due
to parsing or tagging errors (e.g., an instance of the plural noun treasures was tagged as a
third person singular verb form, which in turn prevented the correct analysis from being
found), and partly due to genuine number ambiguity (this/these people).
4.6. Direct comparison of classification results
As we have seen, the initial system performs relatively poorly on sentences containing a
single error (see Sect. 4.3), whereas specialised classifiers for specific error types can perform well. In this section, we present a more direct comparison between the two approaches
by evaluating performance on sentences containing exactly one error of a type for which
a specialised classifier has been developed. In addition to a/an and this/these confusions,
spelling errors were added to the set, partly to compare the naı̈ve machine learning approach with simplistic dictionary look-up, partly to be able to evaluate the other classifiers’
false-positive rate.
General performance for the initial naı̈ve Bayes classifier is reported in Fig. 4.2–4.4; more
specific data in the form of confusion matrices for the three types of error under consideration here can be found in Fig. 4.10.
4.6.1. Individual classifiers independently
Fig. 4.11 shows the results obtained using a specialised classifier for each individual error
type. This particular evaluation method relies upon the assumption that the potential type
of error is known a priori, which is clearly not true in general. We have therefore chosen
not to quote overall performance, since this would have been purely artificial.
As expected, determiner-form errors were detected reliably: only one occurrence of *a
honest slipped through, and an instance of *a English which had not been marked up by
the annotators was correctly flagged as incorrect.
The results for determiner-agreement errors appear less conclusive, but the somewhat low
recall has an explanation: whilst lack of agreement between a noun and this/these is the
predominant error, the category also comprises noun-agreement errors with that/those, as
well as other agreement errors, including those involving pronoun–antecedent agreement
in gender (his/her) and number (its/their).
Spelling mistakes were identified using a list of words derived from a small Oxford dictionary (Mitton 1986) to which have been added 50 frequent words, mostly neologisms like
68
chapter 4. binary sentence classification
Classification
Correct Incorrect
Determiner-form error
Corrected
One error
44
35
13
22
Determiner-agreement error
Corrected
One error
35
10
3
28
Spelling mistake
Corrected
One error
1,421
577
374
1,218
Corrected (Σ)
One error (Σ)
1,500
622
390
1,268
Figure 4.10. Confusion matrices for the initial system (the naı̈ve Bayes classifier for which overall accuracy
is reported in Fig. 4.2). The columns ‘correct’ and ‘incorrect’ indicate, for each set of corrected/incorrect
sentences, the number of sentences that were classified as being either correct or incorrect. Each confusion
matrix shows how well the classifier is able to distinguish between sentences containing one error on the one
hand and their corrected counterparts on the other hand. The bottom matrix is the sum of the three others,
indicating the total performance on these three error types, which can also be expressed as 76% precision and
67% recall.
Classification
Correct Incorrect
Determiner-form classifier
Corrected
One determiner-form error
55
1
0
54
Determiner-agreement classifier
Corrected
One determiner-agreement error
38
19
0
19
1,747
381
48
1,414
Spelling-mistake classifier
Corrected
One spelling mistake
Figure 4.11. Confusion matrices for the individual classifiers. The set of sentences is the same as in Fig. 4.10,
but instead of using a general classifier, a specialised one is used for each type of error. Each classifier only deals
with sentences containing the type of error that it has been designed to detect, as well as corrected versions of
those (i.e., the classifiers do not yet have to handle sentences containing other types of error.)
website and fax or alternative spellings like cafe without an accent and organisation with
an s. Lower-case words not in the list are considered to be incorrect, whereas words containing upper-case letters are not checked at all; consequently, a few correct words (mostly
somewhat rare words or non-Oxford variants) were flagged as incorrect, whereas a more
important number of words written with an initial capital (only a minority of which are
proper nouns) or entirely in upper case went unnoticed. This highly unoptimised system
actually performed better than the general classifier, which is probably due to the fact that
the latter can only detect misspelt words that occur in the training data.
4.6. direct comparison of classification results
69
Classification
Correct Incorrect
Determiner-form classifier
Corrected
One error
1,887
1,835
3
55
Determiner-agreement classifier
Corrected
One error
1,886
1,865
4
25
Spelling-mistake classifier
Corrected
One error
1,840
474
50
1,416
Corrected (Σ)
One error (Σ)
1,833
401
57
1,489
Figure 4.12. Confusion matrices for the individual classifiers on the full test set. As opposed to in Fig. 4.11,
each classifier is given the full set of sentences for consideration and is supposed to pick out the sentences that
contain the type of error on which it specialises. Here, ‘correct’ only means that the classifier in question did
not find an error, not necessarily that the entire sentence should be regarded as impeccable, as there could well
be other types of error detected by other classifiers. A few sentences are flagged as incorrect by more than one
classifier, which in particular explains that the total number of incorrect sentences correctly identified as such
is lower than the sum of the corresponding numbers provided by each of the individual classifiers. In summary,
the system gives 79% recall with 96% precision on the test set.
4.6.2. Individual classifiers together
What happens when the individual classifiers are let loose on the entire test set? A higher
number of false positives can be expected, but as shown in Fig. 4.12, this effect is relatively
modest. Sentences within which an error is found by at least one of the classifiers are
considered to be incorrect; the remainder are taken to be correct. Combining the evidence
in this way gives a system with an F-score of 87 per cent on the test set, as compared to 71
per cent for the initial system.
4.6.3. Specialised classifiers providing additional features
Nothing prevents us from using discriminative information obtained from the individual
classifiers as features for the initial machine-learning system. We tried to add a feature
indicating whether or not a spelling mistake was detected in order to see whether additional
information can actually be beneficial without any other changes to the system. (Features
corresponding to the other classifiers could of course have been added as well, but given
the relative small number of errors affected, doing so would not have had much influence
on the overall performance.)
As can be seen from Fig. 4.13, the performance in terms of accuracy on the test set for the
three machine learning algorithms we tried increased with 1–1.5 percentage points when
this new feature was added. For the naı̈ve Bayes classifier, the improvement corresponds to
70
chapter 4. binary sentence classification
Classifier
Naı̈ve Bayes
Balanced winnow
Maximum entropy
Training data
Test data
80.47%
95.33%
77.80%
71.31%
71.39%
72.41%
Figure 4.13. Accuracy on an experiment similar to the one from Fig. 4.2, the only difference being the addition
of a new binary feature indicating likely misspellings. Each classifier performs statistically significantly better
when this feature is added. The difference in accuracy between the naı̈ve Bayesian and the balanced winnow
classifiers is not statistically significant, whereas the one between either of them and the maximum entropy
classifier is.
an additional 391 sentences correctly classified, as compared to a reduction of misclassified
sentences by 518 when the specialised classifier was used directly. Given the amount of
noise and the number of features used by the classifier, this effect of adding one additional
feature seems rather encouraging.
4.7. Conclusion
An error-annotated corpus provides sentences known to be incorrect; when corrections
are provided, like in the clc, correct and incorrect versions of the same sentence can be
extracted, which makes it easier for a machine learning algorithm to identify salient characteristics of correct and incorrect sentences, respectively. As a useful addition to individual
words and n-grams, a parser provides more general features (parts of speech) as well as
more linguistically motivated ones (grammatical relations).
We have seen that a classifier trained on correct and incorrect sentence pairs can give a binary sentence classification accuracy of over 70 per cent. As one might expect, the system
is more successful at handling morphological and typographical errors than syntactic ones,
and the lowest success rate is observed for verb tense errors, which are likely to depend
on extra-sentential context. The extent to which errors can be detected reliably in individual sentences will be discussed further in Chapter 7, as will the problem of inconsistent
annotation in the corpus.
Our analysis of the classification performance also showed that the system was able to
detect highly deviant sentences with high accuracy, but was less successful with sentences
containing but a single error. It would be interesting to see whether excluding sentences
with more than one error or with errors spanning several words from the training data
might be beneficial, although this might exclude certain types of errors which seldom appear on their own. Another solution, and the one we have presented as successful, consists
in detecting specific types of error directly. Such expert detectors may be used directly or
used as an additional source of information for sentence classification.
Chapter 5.
Synthetic errors
e saw in the previous chapter that pairs of correct and incorrect versions of sentences extracted from a corpus can be used for training and evaluation of a system
that distinguishes between correct and incorrect sentences. A learner corpus like the clc is
a good source of errors for a classifier aimed at detecting human errors, in particular the
ones committed by foreign learners of English, since it contains real errors produced by this
demographic group. An alternative is to use ungrammatical data generated automatically
from a corpus of correct sentences, more readily available and generally larger than an annotated learner corpus. However, such artificial data are of little use if the errors are not
sufficiently similar to naturally occurring ones. In this chapter, we shall have a look at a
tool called GenERRate.23
W
In order for the synthetic error corpus to be useful in an error detection system, the errors
that are introduced need to resemble those that the system aims to detect. Thus, the process
is not without some manual effort: knowing what kind of errors to introduce requires the
inspection of real error data, a process similar to error annotation. Once the error types
have been specified, though, the process is fully automatic and allows large error corpora
to be generated. If the set of well-formed sentences into which the errors are introduced is
large and varied enough, it is possible that this will result in ungrammatical sentence structures which learners produce but which have not yet been recorded in the smaller naturally
occurring learner corpora. To put it another way, the same type of error will appear in lexically and syntactically varied contexts, which is potentially advantageous when training a
classifier. (The procedure described in this chapter will result in a monolithic error corpus
with no concept of different backgrounds, different genres and so on, although it would
of course be possible to create a series of synthetic corpora with different characteristics
insofar as an appropriate description can be given for each.)
23
) The requirements of such a tool, in particular the types of error it should include, were discussed extensively
with Jennifer Foster, and the implementation of GenERRate is mostly due to her.
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chapter 5. synthetic errors
5.1. Earlier use of artificial error data
Artificial errors have been employed previously in targeted error detection. Sjöbergh &
Knutsson introduced split-compound errors and word-order errors (2005) into Swedish
texts and used the resulting artificial data to train their error detection system. These two
particular error types were chosen because they are frequent errors amongst non-native
Swedish speakers whose first language does not contain compounds or has a fixed word
order. They compared the resulting system to three Swedish grammar checkers, and found
that their system had higher recall, for certain types of error almost twice as high as other
systems, at the expense of lower precision when tested on a collection of errors, although
it could not compete with state-of-the-art systems on more realistic data with lower error
rates. Brockett, Dolan & Gamon introduced errors involving mass/count noun confusions into English news-wire text and then used the resulting parallel corpus to train a
phrasal smt system to perform error correction (2006). This system was only tested on a
collection of errors, and found to give much higher recall than Microsoft Word’s system,
again at the expense of lower precision. Lee & Seneff automatically introduced verb form
errors (subject–verb agreement errors, complementation errors and errors in a main verb
after an auxiliary) (2008b), parsed the resulting text and examined the parse trees produced.
The ‘disturbances’ in the parse trees observed for the sentences into which errors had been
introduced were then used as indicative of error.
Both Okanohara & Tsujii (2007) and Wagner, Foster & Genabith (2007) attempted
to learn a model which discriminates between grammatical and ungrammatical sentences,
and both used synthetic negative data obtained by distorting sentences from the bnc. The
methods used to distort the bnc sentences are, however, quite different: Okanohara &
Tsujii generated ill-formed sentences by sampling a probabilistic language model and ended
up with ‘pseudo-negative’ examples which resemble machine translation output more than
language produced by humans:
We know of no program, and animated discussions about prospects for trade barriers or regulations on the rules of the game as a whole, and elements of decoration of this peanut-shaped to
priorities tasks across both target countries.
Indeed, machine translation is one of the applications of their resulting discriminative language model, which is able to distinguish between correct and incorrect sentences with an
accuracy of 74 per cent (given a set of incorrect sentences consisting of this type of highly
deviant constructions). Wagner, Foster & van Genabith introduced grammatical errors of
the following four types into bnc sentences: context-sensitive spelling errors, agreement
errors, errors involving a missing word, and errors involving an extra word. All four types
were considered equally likely and the resulting synthetic corpus contains errors that look
like the kind of slips that would be made by native speakers (e.g., repeated adjacent words)
as well as errors that resemble learner errors (e.g., missing articles). They reported a drop
5.2. error generation tool
73
in accuracy for their classification methods when applied to real learner texts as opposed to
held-out synthetic test data (Wagner, Foster & Genabith 2009), reinforcing the earlier
point that artificial errors need to be tailored for the task at hand.
Artificial error data has also been put into use in the automatic evaluation of error detection
systems, as exemplified by Bigert’s use of a tool called Missplel to generate spelling errors
used to evaluate a context-sensitive spelling checker (2004). Furthermore, the performance of general-purpose nlp tools such as part-of-speech taggers and parsers in the face
of noisy ungrammatical data has been automatically evaluated using artificial error data.
Since the features of machine-learned error detectors are often part-of-speech n-grams or
word–word dependencies extracted from parser output (cf., e.g., De Felice & Pulman
2008), it is important to understand how part-of-speech taggers and parsers react to particular grammatical errors. Bigert & al. introduced artificial context-sensitive spelling
errors into error-free Swedish text and then evaluated parsers and a part-of-speech tagger
on this text using their performance on the error-free text as a point of reference (2005).
Similarly, Foster investigated the effect of common English grammatical errors on two
widely-used statistical parsers by means of distorted treebank trees (2007), using the procedure described by Wagner, Foster & Genabith to introduce errors into the treebank
sentences (2007, 2009).
Finally, negative evidence in the form of automatically distorted sentences has been used in
unsupervised learning. Smith & Eisner generated negative evidence for their contrastive
estimation method by moving or removing a word in a sentence (2005a, 2005b). Since the
aim of this work was not to detect grammatical errors, there was no requirement to generate
the kind of negative evidence that might actually be produced by either native or non-native
speakers of a language. The negative examples were used to guide the unsupervised learning
of a part-of-speech tagger and a dependency grammar.
We can conclude from this survey that synthetic error data has been used in a variety of
nlp applications, including error detection and evaluation of error detectors. In the next
section, we describe an automatic error generation tool, which has a modular design and
is flexible enough to accommodate the generation of the various types of synthetic data
described above.
5.2. Error generation tool
GenERRate is an error generation tool which accepts as input a corpus and an error analysis
file consisting of a list of error types and produces an error-tagged corpus of syntactically
ill-formed sentences. The sentences in the input corpus are assumed to be grammatically
well-formed. GenERRate is implemented in Java and is available as a download for use by
74
chapter 5. synthetic errors
Error
Deletion
Insertion
Move
DeletionPOS
InsertionFromFileOrSentence
MovePOS
DeletionPOSWhere
Subst
SubstMovePOSWhere WordConfusion
SubstWrongForm
InsertionPOS
InsertionPOSWhere
SubstWordConfusionNewPOS
SubstSpecificWordConfusion
Figure 5.1. GenERRate error types.
other researchers.24
5.2.1. Supported error types
Error types are defined in terms of their deviance, that is, in terms of the operations (deletion, insertion, move and substitution) which are applied to a well-formed sentence to
make it ill-formed and which can be reversed to correct the error. As well as being a popular classification scheme in the field of error analysis (James 1998), it has the advantage
of being theory-neutral. This is important in this context since it is hoped that GenERRate will be used to create negative evidence of various types, be it learner-like grammatical
errors, native-speaker slips or more random syntactic noise. GenERRate is intended to
be easy to use for anyone working in linguistics, applied linguistics, language teaching or
computational linguistics.
The inheritance hierarchy in Fig. 5.1 shows the error types that are supported by GenERRate. The error types can be briefly described as follows:
1. Errors generated by removing a word
a) DeletionError: Remove a randomly selected word.
b) DeletionPOSError: Extends DeletionError by allowing a specific part of speech
to be specified.
c) DeletionPOSWhereError: Extends DeletionPOSError by allowing left and/or
right context to be specified (part-of-speech tag or start/end to indicate beginning/end of sentence).
24
) http://www.computing.dcu.ie/∼jfoster/resources/genERRate.html
5.2. error generation tool
75
2. Errors generated by inserting a word
a) InsertionError: Insert a randomly chosen word at a random position in the
sentence. The word is chosen either from the sentence itself or from a word list,
and this choice is also made at random.
b) InsertionFromFileOrSentenceError: Extends InsertionError by allowing to specify whether to choose a word from the sentence or from a word list.
c) InsertionPOSError: Extends InsertionFromFileOrSentenceError by allowing the
part of speech of the inserted word to be specified.
d) InsertionPOSWhereError: Extends InsertionPOSError by allowing left and/or
right context to be specified.
3. Errors generated by moving a word
a) MoveError: Move a randomly selected word to a random position.
b) MovePOSError: Extends MoveError by allowing a specific part of speech to be
specified.
c) MovePOSWhereError: Extends MovePOSError by allowing the change in position to be specified in terms of direction and number of words.
4. Errors generated by substituting a word
a) SubstError: Replace a random word by a word chosen at random from a word
list.
b) SubstWordConfusionError: Extends SubstError by allowing the part of speech
to be specified (same part of speech for both words).
c) SubstWordConfusionNewPOSError: Extends SubstWordConfusionError by allowing two different parts of speech to be specified (i.e., a word of one part of
speech replaces a word of another part of speech, both explicitly indicated).
d) SubstSpecificWordConfusionError: Extends SubstWordConfusionError by allowing specific words to specified (e.g., the substitution of be for have).
e) SubstWrongFormError: Extends SubstError by allowing a substitution with
a different form of the same word to be specified. Currently implemented
substitutions include noun number (word/words), verb number (write/writes),
tense (write/wrote), adjective form (big/bigger) and adjective/adverb confusion
(quick/quickly). Note that only this error type would require adaptation to be
useful for another language (at the moment, only English morphology is supported).
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chapter 5. synthetic errors
5.2.2. Input corpus
The corpus of well-formed sentences that is supplied as input to GenERRate must be split
into sentences. It does not have to be part-of-speech tagged, but it will not be possible
to generate many of the errors if it is not. GenERRate has been tested using two part-ofspeech tagsets: the Penn Treebank tagset (Santorini 1990) and the claws tagset (Garside
1987; see Appendix c).
The error analysis file specifies the errors that GenERRate should attempt to insert into
the sentences in the input corpus. A toy example with the Penn tagset might look like the
following:
subst,word,an,a,0.2
subst,NNS,NN,0.4
subst,VBG,TO,0.2
delete,DT,0.1
move,RB,left,1,0.1
The first line is an instance of a SubstSpecificWordConfusionError, the second and third
are instances of the SubstWrongFormError type, the fourth is a DeletionPOSError, and
the fifth is a MovePOSWhereError. The number in the final column specifies the desired
proportion of the particular error type in the output corpus and is optional; however, if it is
present for one error type, it must be present for all. The overall size of the output corpus
is supplied as a parameter when running GenERRate.
5.2.3. Error generation
When frequency information is not supplied in the error analysis file, GenERRate iterates
through each error in the error analysis file and each sentence in the input corpus, tries to
insert an error of this type into the sentence and writes the resulting sentence to the output
file together with a description of the error. GenERRate includes an option to write the
sentences into which an error could not be inserted and the reason for the failure to a log
file. When the error analysis file does include frequency information, a slightly different
algorithm is used: for each error, GenERRate selects sentences at random from the input
file and attempts to generate an instance of that error until the desired number of errors
has been produced or the set of input sentences has been exhausted.
5.3. Classification experiments
In our original GenERRate paper, results from two binary classification experiments using
synthetic training data were reported (Foster & Andersen 2009). Our aim was not so
5.3. classification experiments
77
CLC
GenERRate
<w>availablejj position|position availablejj </w>
<rj>unknownjj |strangejj </rj>
<rt>ofio |forif </rt>
<rn>housennl1 |buildingnn1 </rn>
recommendvv0 <ua>youppy |</ua> theat
withiw <md>|anat1 </md> apologynn1
move JJ
subst JJ
subst IF IO
subst NN1 NNL1
insert file VV0 PPY AT
delete IW AT1 NN1
<fn>complaintnn1 |complaintsnn2 </fn>
<fv>toto understandvv0 |understandingvvg </fv>
<agn>brainnn1 |brainsnn2 </agn>
<agd>that|those</agd>
<agv>is|are</agv>
<dy>properjj |properlyrr </dy>
<da>my|mine</da>
subst NN2 NN1
subst VVG TO
subst NN2 NN1
subst word those that
subst word are is
subst RR JJ
subst word mine my
Figure 5.2. Examples of errors from the clc with corresponding GenERRate error descriptions. The upper
part shows word-order errors, insertions, omissions and lexical replacements, whereas morphological errors
are shown in the lower part.
much to improve classification performance as to test the GenERRate tool, to demonstrate
how it can be used, and to investigate differences between synthetic and naturally occurring
datasets. Whereas synthetic errors generated by our tool were successfully shown to be
superior to (i.e., closer to real errors than) previously used synthetic errors and thus more
useful as training data, our attempt to create a large error corpus inspired by the clc and
use it in a binary sentence classification task, similar to the one described in the previous
chapter, was less conclusive. This section describes this experiment in detail.
5.3.1. Setup
We attempted to use GenERRate to insert errors into corrected clc sentences. In order to
do this, we needed to create a clc-specific error analysis file, which was done automatically
by extracting erroneous part-of-speech trigrams from the error-annotated clc sentences
and encoding them as GenERRate errors (cf. Fig. 5.2). This resulted in approximately
13,000 errors of the following types: DeletionPOSWhereError, InsertionPOSWhereError,
MovePOSError, SubstWordConfusionError, SubstWordConfusionNewPOSError, SubstSpecificWordConfusionError and SubstWrongFormError. Frequencies were extracted, and errors occurring only once were excluded.
We trained a series of naı̈ve Bayesian classifiers using different combinations of real and
synthetic errors in different quantities, including the following three classifiers: The first
was trained on corrected clc sentences (the grammatical section of the training set) and
original clc sentences (the ungrammatical section). The second classifier was trained on
corrected clc sentences and the sentences generated from the corrected clc sentences using
GenERRate (we call these ‘faux-clc’). The third was trained on corrected clc sentences
78
chapter 5. synthetic errors
Feature
Examples
Word
Word bigram
Part of speech
Part-of-speech bigram
Part-of-speech trigram
a; thought; occur+ed
a thought; thought occur+ed
at1; nn1; vvd
at1 nn1; nn1 vvd
at1 nn1 vvd
Figure 5.3. Overview of the features used. The examples are taken from the fragment aat1 thoughtnn1 occurredvvd .
Training data
Correct/incorrect
Correct/faux
Correct/incorrect+faux
Confusion matrix
Correct
Incorrect
Correct
Incorrect
Correct
Incorrect
Correct sentences
Incorrect sentences
34,814
26,456
8,825
18,917
35,935
33,233
7,704
12,140
36,110
28,058
7,529
17,315
Precision
Recall
Accuracy
69.7%
42.6%
61.3%
62.0%
30.7%
55.1%
69.7%
38.2%
60.0%
Figure 5.4. Classification performance on the test data for different training sets.
and a 50/50 combination of clc and faux-clc sentences. In all experiments, the grammatical section of the training data contained 438,150 sentences and the ungrammatical
section 454,337. The classifiers were tested on a held-out section of the clc containing
43,639 corrected clc sentences and 45,373 original clc sentences. To train the classifiers,
the mallet implementation of naı̈ve Bayes was used.We used a subset of the features mentioned in the previous chapter (see Fig. 4.1), namely word unigrams and bigrams, as well
as part-of-speech unigrams, bigrams and trigrams, illustrated in Fig. 5.3; these are features
that can be constructed directly from the faux-clc sentences as output by GenERRate, thus
avoiding the need for parsing.
5.3.2. Results
The results of the clc classification experiment are presented in Fig. 5.4. There is a 6.2 per
cent drop in accuracy when we move from training on original clc sentences to artificially
generated sentences, which is somewhat disappointing since it means that we have not completely succeeded in replicating the clc errors using GenERRate. Most of the accuracy
drop is on the ungrammatical side: the correct/faux model classifies more incorrect clc
sentences as correct than the correct/incorrect model. One reason for this is that certain
frequent types of error are not included in the error analysis file and cannot be described
within the current framework which largely relies on part-of-speech tags: the corrected
clc sentences used to generate the faux-clc set were tagged with the claws tagset, and although more fine-grained than the Penn tagset, it does not, for example, make a distinction
between mass and count nouns, a common source of error. Another important reason for
the drop in accuracy is the recurrent spelling errors which occur in the incorrect clc test
5.4. limitations of generrate
79
set but not in the faux-clc test set. It is promising, however, that much of the performance
degradation is recovered when a mixture of the two types of ungrammatical training data
is used, suggesting that artificial data could be used to augment naturally occurring training sets. Unfortunately, faux-clc data actually gives a small decrease in performance when
added to the full set of incorrect sentences found in the clc. It would be interesting to see
whether certain types of artificially generated errors are responsible for this deterioration,
whereas others might perhaps be beneficial; furthermore, additional synthetic training data
may be more useful in combination with a more extensive feature set, in which case the
data sparsity problem will be more acute.
5.4. Limitations of GenERRate
The following three limitations, which became apparent to us during the classification experiments, illustrate some of the issues that make the task of generating synthetic error data
non-trivial.
5.4.1. Sophistication of the error specification
There are some coverage issues with GenERRate, some of which are due to the simplicity
of the supported error types. When linguistic context is supplied for deletion or insertion errors, it takes the form of the part of speech of the words immediately to the left and/or right
of the target word. Lee & Seneff analysed preposition errors made by Japanese learners
of English (2008a) and found that a greater proportion of errors in argument prepositional
phrases (look at him) involved a deletion than those in adjunct prepositional phrases (came
at night). The only way for such a distinction to be encoded in a GenERRate error analysis
file is to allow parsed input to be accepted. This brings with it the problem that parsers
are less accurate than part-of-speech taggers, but it may still be preferable to let the system
make a certain number of mistakes than not to be able to specify the conditions for error
insertion because the representation is inadequate. A more significant improvement would
be to make use of WordNet synonym sets or another source of semantic similarity in order
to choose the new word in substitution errors, knowing that humans hardly ever substitute
one word for another without there being some sort of connection between them.
5.4.2. Covert errors
A covert error is an error that results in a syntactically well-formed sentence with an interpretation different from the intended one. Covert errors are a natural phenomenon,
occurring in real corpora. Lee & Seneff gave the example I am preparing for the exam,
which has been annotated as erroneous because, given its context, it is clear that the person
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chapter 5. synthetic errors
meant to write I am prepared for the exam (2008b). The problems lie in deciding what
covert errors should be handled by an error detection system and how to create synthetic
data which gets the balance right.
Covert errors can be produced by GenERRate as a result of the sparse linguistic context
provided for an error in the error analysis file. An inspection of the generated errors
shows that some error types are more likely to result in covert errors. An example is
the SubstWrongFormError when it is used to change a noun from singular to plural. This
results in the sentence But there was no sign of Benny’s father being changed to the wellformed but more implausible But there was no sign of Benny’s fathers. The next version of
GenERRate should include the option to change the form of a word only when it appears
in a certain context.
In the design of GenERRate, particularly in the design of the SubstWrongFormError type,
the decision was made to exclude tense errors because they are likely to result in covert
errors (e.g., she walked/walks home), but in doing so we also avoid generating examples
like this one:
(15) When I was a high school student, I *go/went to bed at one o’clock.
These tense errors are common in learner data and their omission from the faux-clc training set is one of the reasons why the performance of this model is inferior to the real-clc
model.
5.4.3. More complex errors
The learner corpora contain some errors that are corrected by applying more than one
transformation. Some are handled by the SubstWrongFormError type (I spend a long time
*to fish / fishing), but some are not (she is one of *reason / the reasons I became interested
in English).
5.5. Conclusion and further work
We have presented GenERRate, a tool for automatically introducing syntactic errors into
sentences and shown how it can be used to create synthetic training data for grammatical
error detection research. Although we have focused on the binary classification task, we
also intend to test GenERRate in targeted error detection. Another avenue for future work
is to explore whether GenERRate could be of use in the automatic generation of language
test items (cf., e.g., Chen, Hsien-Chin & Chang 2006). Our immediate aim as far as
GenERRate development is concerned is to produce a new version which tackles some of
the coverage issues highlighted by our experiments.
5.5. conclusion and further work
81
Although GenERRate can provide training data that is more useful than previous collections of synthetic errors (Foster & Andersen 2009), it does still not rival a real error
corpus. More experiments are needed to see whether specific types of synthetic errors can
constitute a useful supplementary source of incorrect examples, perhaps with the addition
of more sophisticated error generation mechanisms as suggested previously.
Finally, it should be mentioned that the lack of a generally available error corpus for use as
test data acts as a significant impediment to the comparison of different systems’ respective
performance. A synthetic test corpus would however have to be closely modelled on the
errors and error distribution in a real corpus lest it constitute an artificial yardstick of little
relevance to the actual task, and thus necessarily reveal some essential characteristics of the
model corpus; publishing a small part of the clc (or a similar corpus) might be a better
solution.
Chapter 6.
Replacement errors
ata sparsity is a problem that permeates all empirical or corpus-based approaches to
natural language processing, and it exhibits itself quite prominently in the task of
error detection, since perfectly correct but for some reason unusual sentences, including the
ones a more or less specialised language generation system may choose to avoid, should
not be flagged as incorrect just because they happen to be statistically improbable. In
this chapter, we shall see how data from the bnc combined with WordNet and clustering
methods can provide the necessary evidence for detecting two types of lexical replacement
errors, namely adjectival and prepositional choice errors.
D
6.1. Adjectival choice errors
Unlike glaring syntax errors like failing verb–noun agreement, adjectival choice issues must
typically be ascribed to lack of idiomaticity rather than infringement of a clear grammatical
rule. This suggests that the information required to detect infelicitous adjective–noun pairs
can be extracted directly from a reference corpus, at least to the extent to which the native
speaker’s appreciation of idioms correlates with actual usage. One way of doing this, as
well as the results obtained for a small set of adjectives, will be described in the following.
We posit that an inappropriately chosen adjective tends to be semantically close to or at
least related to the one that should have been chosen. For instance, brisk walk is probably
more idiomatic in some sense than nice walk, but we would not want to suggest the former
as a replacement for the latter since they mean quite different things.
On the other hand, fast walk is mostly synonymous with — and could, perhaps advantageously, be replaced by — brisk walk. Proposing more idiomatic alternatives to perfectly
acceptable expressions can be useful for some applications, but not if our aim is to detect
ungrammatical constructions proper. Getting this right is at least in part a question of
appropriate thresholds.
84
chapter 6. replacement errors
†
†
great
large
wide
high
broad
loud
good
144
122
33
14
10
9
8
†
†
†
†
†
long
heavy
terrible
tall
major
tremendous
considerable
6
5
4
4
4
3
3
†
†
†
extensive
vast
sufficient
significant
serious
popular
important
2
†
†
†
huge
greatest
grand
favourite
best
bad
Figure 6.1. Adjectives replacing big in the clc, sorted by frequency (1 not explicitly indicated). The ones that
do not belong to the same synonym set as big in WordNet are marked with a dagger.
We make the assumption that all ‘significant’ idiomatic adjective–noun combinations occur
‘frequently’ in a big corpus like the British National Corpus (bnc). This means in particular
that any adjective will be considered appropriate in conjunction with a rare noun and,
furthermore, that any adjective in a set of confusables, typically quasi-synonyms, likewise
will be considered appropriate if none of them co-occur ‘frequently’ with a given noun,
since there is no clear evidence either way.
In the following, we shall investigate the feasibility of automatically detecting infelicitous
instances of the adjective big found in the Cambridge Learner Corpus (clc). This happens
to be the most commonly misused adjective in the corpus (383 errors), and the class of
magnitude adjectives has also been discussed previously (Copestake 2005).
6.1.1. Semantic relatedness
Fig. 6.1 shows the full set of adjectives suggested as adequate replacements for big in different contexts. As one might expect, a small set of fairly common adjectives accounts for a
large proportion of the corrections, and most of these are indeed semantically close to big.
WordNet’s synonym set confirms our intuition: it may well be that only 14 out of 27 words
appear in the same set as big, but these cover more than 94 per cent of the instances. This
suggests that synonym sets like the ones defined by WordNet might be used to approximate
real confusion sets, which will typically be useful for less common adjectives.
In 319 of the 383 instances (83%), the parser successfully identifies the adjective as directly
modifying a noun. As can be seen from the examples below, the correction provided by
the annotator tends to keep the idea of magnitude, though this is less obvious when the
appropriate scale is qualitative, as in the last two examples.
(16) a. a *big/large number of tourists
b. beautiful views and a *big/wide range of facilities
c. a *big/broad experience of working with children
d. cause *big/tremendous disasters we can’t imagine
6.1. adjectival choice errors
big
broad
considerable
extensive
good
great
*
*
*
85
grand
heavy
high
huge
important
large
**
**
**
long
loud
main
major
numerous
strong
tall
terrible
tremendous
wide
Figure 6.2. Confusion set for big. This set includes, apart from the word itself, all the adjectives that are
proposed at least twice as a replacement for big in the clc, as well as some quasi-synonyms proposed only
once (marked with an asterisk) or not at all (marked with a double asterisk). The choice of a confusion set
which does not reflect the clc perfectly is intended to make the task of selecting the correct word more realistic.
e. a *big/wide variety of hot meals every day
f. quite friendly with a *big/good sense of humour
g. My *big/favourite hobby is horse-riding.
In the following experiments, our test set comprises these 319 occurrences of big represented in terms of incorrect adjective (i.e., *big), correct adjective proposed by the annotator
and noun being modified (e.g., *big, large, number).
6.1.2. Corpus frequencies vs. annotator judgements
Co-occurrence statistics extracted from the BNC can be used to determine whether the annotators’ preferences in cases where big has been marked up as erroneous agree with evidence of usage: We used the Robust Accurate Statistical Parsing system (rasp) to identify
adjective–noun relations in the bnc and considered the set of adjectives listed in Fig. 6.2 as
semantically close to and thus potentially confusable with big. We then ranked the adjectives in the confusion set, for each of the nouns in the test set, according to the number of
times each adjective co-occur with the noun in question. (Unlike more ambitious collocation metrics, the frequency/rarity of each word separately is not taken into account, which
effectively gives a bias towards more common adjectives in our case.)
The naı̈ve strategy of choosing the most common adjective for each noun gave the following
result when applied to the test set of 319 instances of inappropriate use of big: in 223 cases,
the most common adjective in the bnc is also the one chosen by the annotators; in 88
cases, the annotators and the bnc differ (sometimes because the annotators have chosen a
correction outside our confusion set), but neither ends up with big; in 6 cases, no evidence
can be found as the noun does not co-occur with any of the adjectives in the confusion
set; and in the remaining 2 cases, big actually turns out to be the most commonly chosen
adjective. Obviously, there will often be more than one possible correction, so the fact that
the bnc and the annotators sometimes suggest different ones is not necessarily a problem.
86
chapter 6. replacement errors
6.1.3. Error detection and correction
In a more realistic scenario, the system will have to identify incorrect instances of big amidst
a considerably larger number of correct ones. (Even in the clc, the adjective is actually
used correctly 95 per cent of the times it occurs.) By using the method previously applied
to incorrect instances, we were able to extract 5,810 purportedly correct adjective–noun
pairs. We should then like the system to detect as many of the 319 incorrect instances as
possible without mistakenly condemning the correct ones.
The bnc provides a good account of correct English usage, but should not be taken as an
infallible gold standard. In particular, its breadth of coverage inevitably leads to inclusion
of marginal usage, which in our case implies that an adjective–noun combination which
occurs only twice or thrice in the bnc should not necessarily be regarded as a strong collocation. Perhaps even more importantly, a sizeable difference in frequency is required for one
adjective to be considered as clearly ‘better’ than another in a given context. (Excluding
certain genres, such as poetry and unscripted speech, can be expected to filter out many
expressions that are not really part of the standard language, but this approach was not
tested.)
We started with a rather conservative threshold, considering an adjective as a possible
alternative to big only when the adjective–noun combination occurs at least 100 times
more frequently in the bnc than the corresponding big–noun combination, or at least 100
times if big–noun cannot be found at all. Choosing the most frequent adjective in each case
then permitted 90 out of the 319 incorrect occurrences of big to be correctly identified as
such (28% recall), and the system provided the same correction as the annotator in 79 cases.
On the other hand, 25 of the correct occurrences were also signalled as incorrect, including
the following examples:
(17) a. *big/wide range
b. *big/large number
c. *big/wide variety
As the attentive reader will have noticed, these corrections are identical to those provided
by the annotators for the same constructions occurring elsewhere in the corpus (cf. Ex. 16),
and our preliminary conclusion is that most, if not all, of these cases really should be
regarded as incorrect, and that mere oversight has led to their not being annotated accordingly.
Lowering the threshold to 50 allowed the system to detect 152 errors (48% recall) and
correct 132 of them in accordance with the annotators’ prescription. An additional 27
correct instances (52 in total) were also classified as incorrect. It would seem that most of
these really ought to be corrected, but the system may be led astray in contexts where two
adjectives taken from our set of confusables are not actually synonymous:
6.2. prepositional choice errors
87
(18) *big/good fortune
The topic under discussion is monetary wealth, which means that the proposed correction
rather distorts the meaning. Whether or not the original is actually infelicitous may be a
matter of opinion. It is interesting to note, however, that the expression large fortune does
occur in the bnc, albeit much less frequently than good fortune, whereas big fortune cannot
be found at all.
With an even more liberal threshold of 25, the system found 187 errors (59% recall) and corrected 160 successfully, whereas the number of correct occurrences identified as incorrect
rose to 155, including at least a few which should clearly not be considered as incorrect:
(19) *big/great deal
As a matter of fact, big deal occurs 58 times in the bnc, which suggests that this problem
could, at least in part, be alleviated by using a slightly more sophisticated metric, in particular not considering combinations that actually do occur a certain number of times in the
bnc as potentially erroneous.
For this technique to be useful for practical applications, it would have to be extended to
cover other classes of words in addition to the set of adjectives related to big; an immediate extension to the work presented here would be to check whether it applies to other
adjectival confusion sets as well.
6.2. Prepositional choice errors
In the remaining part of this chapter, the particular problem of detecting prepositional
choice errors in a syntactic verb–preposition–noun context will be investigated, using a
model based on the intuition that a preposition is likely to be incorrect if there is overwhelming evidence for another one in the same context. We shall see how co-occurrence
statistics for verb–preposition–noun triples can be accumulated over sets of verbs and nouns
with similar prepositional preferences and thus allow the information present in a corpus
to be exploited more effectively, which gives a significant increase in recall without loss in
precision.
Prepositions fulfil a wide range of linguistic functions, and the choice of a particular preposition in a given context often appears to be governed by complex and elusive criteria which
cannot easily be summarised in such a way as to offer useful guidance to learners of the
language, or even compiled into a potentially large set of rules for use in natural language
generation or machine translation. A useful (albeit not always clear-cut) distinction is the
one between lexical and functional use:
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chapter 6. replacement errors
(20) a. The milk is in the refrigerator.
b. The cat is on the refrigerator.
(21) a. Kimberly relies on her husband.
b. Kimberly believes in his wife.
In Ex. 20, the change of preposition reflects a change in location, whereas the choice of
preposition in Ex. 21 depends entirely on the verb, the preposition itself being essentially
devoid of intrinsic meaning. We make no attempt to distinguish between these two uses in
any formal way, but our focusing on prepositions for which a verbal head and a nominal
dependent can be identified is thought to give a certain bias towards functional use. The
very same challenge regarding prepositional choice may of course exist in phrases where
the head is adjectival (Mrs Kimberley is reliant on her spouse) or nominal (Mr Kimberley’s
belief in his consort), and the method described in the following can be generalised to cover
such cases as well.
The last few years have seen a rising interest in automatic detection of prepositional choice
errors alongside the long-standing research problem of preposition generation or ‘guessing’.
Recent relevant publications include De Felice & Pulman, who trained a classifier using
contextual features such as parts of speech, grammatical relations and semantic categories
(2007, 2009); Tetreault & Chodorow, who used a similar model, with fewer and simpler
features, augmented with heuristic rules (2008); and Gamon & al., who presented a system
incorporating in addition a 5-gram language model (2008).
It would of course be possible to identify incorrectly chosen prepositions without necessarily being able to suggest an appropriate replacement; however, at least partly owing to the
scarcity of negative examples,25 the typical approach, and the one we adopt here, is first
to find either the ideal preposition in a given context or a set of allowable prepositions and
then use this information to gauge the appropriateness of the original. The two are thus
intimately related.
6.3. Model
We used the British National Corpus (bnc) to represent correct use of prepositions. The use
of a balanced corpus like the bnc should give a fairly general model without bias towards
a specific genre. It could of course be useful to adapt it for a specific application, but our
25
) Whereas specimens of correct usage can easily be extracted from standard corpora, prepositions known to
be wrong in a given context can only be found in an annotated error corpus, and a complete list of inappropriate
choices would clearly be an elusive goal.
6.3. model
89
ncmod
ncsubj
iobj
everyone go+ed
pn1
vvd
dobj
to
ii
France
np1
det
this
mc
summer
nnt1
Figure 6.3. Analysis of the sentence Everyone went to France this summer. The words have been lemmatised,
and the parser has added dependency relations (grs). The arrows representing grs are drawn pointing from
head to dependent; the label above the arrow indicates the type of relation.
focus was on that which is incorrect across a wide variety of text types rather than that
which might be unusual or inappropriate for a particular genre.
Once we have chosen a reference corpus, the next question is how to characterise the
context of a preposition, given that we would like to predict prepositional choice based on
the context in which the preposition appears.
(22) Everyone went to France this summer.
In Ex. 22, the choice of to is guided by the words went and France, which conveniently
appear on either side of the preposition.
(23) They all went together on the train.
However, as Ex. 23 shows, immediate linear context is not always particularly helpful:
together and the do not on their own provide particularly strong cues as to what the correct
preposition is. Furthermore, we probably want to carry out lemmatisation in order to
identify, for instance, the past tense went with the corresponding infinitive go.
To deal with these issues, we used a parsed version of the corpus and identified relevant
features based on dependency relations. Fig. 6.3 shows how the sentence from Ex. 22 is
analysed by the rasp system. Whenever there is an iobj relation from a verb to a preposition,
and a dobj relation from the same preposition to a noun, a verb–preposition–noun triple
(v, p, n) can be constructed. For instance, the sentence in Fig. 6.3 might give rise to the
triple (v, p, n) = (go, to, France).
The parser is not always successful at distinguishing between adjuncts and arguments, so it
would arguably be better to include not only arguments (analysed as iobj), but also adjuncts
(analysed as ncmod), especially since the choice of preposition may occasionally alter the
analysis. However, the vast majority of prepositions that can be confidently identified
as modifying a verb are analysed as arguments (95% in our set, based on only the most
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chapter 6. replacement errors
probable analysis given by rasp for each sentence), so this should not have a significant
impact on the results reported. Another solution, in particular if this had been more of a
concern, would have been to consider the probabilistic gr set, incorporating the full set of
possible analyses rather than the best parse only.
We were able to extract over a million distinct triples from the bnc, with frequencies varying
from one to over a thousand. In the following sections, we use the notation ν(v, p, n) to
refer to the number of occurrences of (v, p, n). The ultimate goal is to make use of this
frequency information to detect erroneous use of prepositions.
6.4. Clustering
A naı̈ve approach to error detection would be to consider a triple (v, p, n) to be correct
if it occurs at least once in the bnc, and incorrect if it does not occur. This relies on the
assumption that the bnc can be taken as a gold standard in the sense that no construction
occurring even once should ever be considered as potentially incorrect, which may not be
fully warranted given that the corpus includes slightly non-standard expressions as well as
transcription errors and original printing errors. More importantly, ν(v, p, n) will often be
low or zero even for correct constructions, which makes this approach inapplicable in its
primitive form; in fact, as many as 40 per cent of the triples extracted from the bnc occur
only once, which clearly suggests that there must be a large number of correct combinations,
many of which cannot be expected to have been seen before, even in a large corpus.
Hence the idea of clustering: if we can ascertain that a noun n belongs to a cluster N of
nouns which all behave in a similar fashion with respect to prepositional preferences, and
that v likewise belongs to a cluster V of verbs behaving similarly to each other, the following
accumulated frequencies can be calculated:
ν(V, p, n) =
X
ν(v , p, n)
v ∈V
ν(v, p, N) =
X
ν(v, p, n )
n ∈N
ν(V, p, N) =
XX
ν(v , p, n )
(v ,n )∈V×N
Problems of data sparsity can then be mitigated by using such generalised frequencies to
augment or replace specific counts.
Rare words, namely verbs occurring in fewer than 10 triples and nouns occurring in fewer
than 25 triples, were discarded, partly because a minimum of evidence is needed for the
clustering to be meaningful, and partly as an efficiency/comprehensiveness trade-off. These
thresholds gave 4,040 verbs and 5,858 nouns from the bnc to be clustered.
6.4. clustering
91
Obviously relevant features for verb clustering include prepositional co-occurrence counts
P
ν(v, pi ) =
ν(v, pi , n) for each verb v, which can be derived directly from the (v, p, n)
n
P
ν(v, pi , n) for noun clustering.
triples extracted from the bnc, and similarly ν(pi , n) =
v
The set of prepositions (pi ) used as co-occurrence features is fairly complete with 62 prepositions including unto, opposite and aboard, only exceedingly rare ones such as vis-à-vis,
unabbreviated versus and elided ’fore being excluded.
There are many other features of potential relevance, and we had initially planned to add
more, but this small set of 62 features per verb/noun actually turned out to provide the
information needed by the clustering algorithm to construct fairly good clusters.
6.4.1. Non-parametric Bayesian clustering
One problem with many clustering algorithms, which may have contributed to limiting
their use in natural language processing applications, is that they typically require the number of clusters to be known in advance, which is often not possible in realistic scenarios.
In contrast, non-parametric Bayesian models can discover a reasonable number of clusters
based on the data whilst allowing the general level of granularity to be parameterised.
The following clustering experiments26 are inspired by Vlachos, Korhonen & Ghahramani, who used a Dirichlet process mixture model (dpmm) for clustering verbs into
semantic classes using subcategorisation frames (2009). Each instance xi with its characteristic features (in our case, each verb/noun with its characteristic prepositional distribution)
is considered to have been generated by a distribution F with parameter θi ,
xi |θi ∼ F(θi );
the parameters θi are in turn considered to derive from a distribution G,
θi |G ∼ G,
a Dirichlet process with base distribution G0 and dispersion parameter α,
G|α, G0 ∼ DP(α, G0 ).
In other words, the instances xi are drawn from a mixture of component distributions F(θi ),
and each component corresponds to a cluster.
The prior probability of assigning an instance to a particular cluster is proportionate to
the number of instances already assigned to it; in other words, a dpmm exhibits the ‘richget-richer’ property. In addition, the probability that a new cluster be created is dependent
on the dispersion parameter. A popular metaphor to describe a dpmm, which exhibits an
26
) The actual clustering algorithm was implemented by Andreas Vlachos.
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chapter 6. replacement errors
equivalent clustering property, is the Chinese restaurant process: customers (sc. instances)
arrive at a Chinese restaurant with an infinite number of tables (sc. clusters); each customer
sits down at one of the tables, previously occupied or vacant, with popular tables attracting
more customers.27
The prior for assigning an instance xi to either an existing component z or to a new one z
conditioned on the other component assignments (denoted by z−i ) is given by
p(zi = z|z−i ) =
and
p(zi = z |z−i ) =
n−i,z
N−1+α
α
,
N−1+α
where N is the total number of instances, n−i,z is the number of instances assigned to component z excluding instance xi , and α is the dispersion parameter. A clustering is generated
by assigning more than one instance to the same mixture component. Instances that are
assigned to the same component have equal θi ’s.
The distribution used to model the clusters is the multinomial (F), and the prior used is
the Dirichlet distribution (G0 ), which is the conjugate prior of the multinomial and therefore allows analytic integration over the parameters of the multinomial. Following Neal,
the component assignments zi of each instance xi are sampled using the following scheme
(2000):
P(zi = z|z−i , xi ) = b p(zi = z|z−i ) F(xi |zi = z, x−i,z , λ),
where b is a normalising factor, λ are the parameters of the Dirichlet prior G0 , and x−i,z are
the instances already assigned to component z (none if we are sampling the probability of
assignment to a new component). This Gibbs sampling method is possible due to the fact
that the instances in the model are interchangeable (i.e., the order in which they are generated is not relevant); in terms of the Chinese restaurant process metaphor, we consider each
instance xi in turn as the last customer to arrive, and he chooses to sit together with other
customers at an existing table or to sit at a new table. As did Navarro & al., who used
the same model to analyse variation between individuals (2006), we sampled the dispersion
parameter α using the inverse Gamma distribution as a prior; only the parameters λ of the
Dirichlet prior had to be set manually.
6.4.2. Evaluation issues
Unsupervised clustering is difficult to evaluate. Recent work has focused on evaluation of
a clustering result given a manually created gold standard containing all the instances assigned to clusters according to the task at hand (Meilă 2007; Rosenberg & Hirschberg
27
) The author is not familiar with any restaurant, Chinese or otherwise, whose customers behave in this
way; the slightly different idea of already overcrowded restaurants that continuously attract new diners, whilst
empty ones struggle to entice passers-by to enter, might be more evocative of the underlying concept.
6.4. clustering
93
2007). While using an external gold standard is a perfectly valid evaluation approach,
it requires substantial human effort, especially if one considers that, unlike annotation of
datasets for classification tasks, assignment of each instance to a cluster interacts with the assignments of other instances. Thus, in cases where the number of instances to be clustered is
large, constructing a comprehensive gold standard for evaluation is usually not considered.
Moreover, the very concept of a gold standard is problematic for ‘unconstrained’ clustering
since there is no well-defined and predetermined set of categories.
A common way of assessing the utility of clustering as a processing step in the pipeline of a
larger system is to look at the effect it has on the system’s performance, a type of analysis we
perform in the context of preposition guessing and correction (see Sect. 6.5); a more direct
evaluation of the clustering quality is nevertheless desirable, not least because it allows the
adequacy of the model (and the effect of different parameter settings) to be assessed at an
earlier stage.
As we have seen, only purportedly correct instances from the bnc were used as input to the
clustering algorithm, but those (even in the form of a held-out set) do not provide the basis
for a good evaluation of the resulting clusters. Manually looking at some of the clusters
may be sufficient to determine whether the clustering is potentially meaningful or minimally
promising, but remains purely qualitative, and there is no such thing as a comprehensive
gold standard for this task. As a partial solution, we made use of erroneous instances
corrected in the clc:
Consider a sentence like the following, where the candidate has written in, which the annotator has corrected to to:
(24) Everyone went *in/to France this summer.
Prepositional choice errors do not typically change the sentence structure28 , so we can
proceed as outlined previously to extract the quadruple (v1 , q, p, n) = (go, in, to, France),
where q is the incorrect preposition and p is the correct preposition. Now, if we find the
quadruple (v2 , p, q, n) = (arrive, to, in, France), we know that v1 (arrive) and v2 (go) do
not belong in the same cluster with respect to their prepositional preferences. Similarly, the
quadruples (v, q, p, n1 ) = (sit, in, on, sofa) and (v, p, q, n2 ) = (sit, on, in, armchair) would
tell us that the nouns n1 (sofa) and n2 (armchair) should not be clustered together.
Using this strong criterion, we can construct a set of verb pairs (and noun pairs) that
should not be clustered together, and these pairs can be used to evaluate the results from
the clustering experiments described in Sect. 6.4.3.
28
) The parser component of the rasp system works on part-of-speech tags rather than words or lemmata, so it
is typically unable to distinguish between prepositions. However, the prepositions for, of and with have unique
tags, so the parse may change when one of these prepositions is involved.
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chapter 6. replacement errors
One might reasonably be concerned about the validity of this evaluation, given that the
rather strong criterion obviously precludes extensive coverage, whereas particular idioms
may cause the clustering algorithm to be penalised unduly for clustering words which really
behave similarly in most contexts (though it is not entirely obvious that words like bed and
futon are as similar as one might at first expect). We argue that a less strict criterion could
easily lead to an evaluation based on words which ‘might reasonably’ belong to different
clusters or ‘should probably not’ be clustered together, which would not be particularly
enlightening. In any case, this method actually gives a couple of thousand verb pairs (and
noun pairs) upon which to base a preliminary evaluation, and we have found the results
from this evaluation to correlate strongly with the performance obtained when the same
clusters are used for preposition guessing and error detection in following experiments,
and also agree roughly with our intuition of cluster quality as gauged by looking at cluster
samples.
6.4.3. Clustering experiments
Feature choice and representation is crucial for unsupervised learning since, unlike in the
case of supervised learning, there are no labelled instances that can provide guidance on
correct interpretation and relative importance of individual features.
As already anticipated, preposition co-occurrence counts (i.e., the number of times each
preposition is used as a modifier of a given verb or the number of times each preposition
is used with a given noun as its object) will be used as features in the following verb and
noun clustering experiments. This choice is justified by our need to discover clusters of
words that have similar behaviour with respect to preposition usage, which eventually can
be used for preposition guessing and correction.
In the experiments reported below, we ran the Gibbs sampler 5 times, each time letting
the sampler go through 100 iterations before we started sampling (‘burn-in’) in order to let
the system reach a state that should not be affected by the initialisation, and then drawing
20 samples from each run with 5 iterations’ lag between samples, values that have been
reported to work well in prior work. The parameters λ of the symmetric Dirichlet prior
was set to 10−4 , a very small value which allows for more flexible cluster formation, since
at inference time the statistics of each new cluster generated is dependent almost exclusively
on the features of the single instance assigned to it. Different values of λ were tried in
preliminary experiments.
From the Gibbs sampler, we obtained a set of 100 samples in each experiment. As explained
in Sect. 6.4, we intended to replace counts of single verbs/nouns with the counts of their
respective containing clusters, which requires a unique set of clusters. While it would be
desirable to average over the clustering samples obtained, this is not possible, since the
clusters in a particular sample cannot be identified with any of the clusters in a different
6.4. clustering
95
sample. A different option would be to identify a particular sample as representative of all
the samples, by measuring the average distance of each sample to all the others, but any
individual sample is likely to contain clusters that are not representative of other samples.
To avoid these issues, we generated a unified clustering from the set of samples by means
of the procedure used for qualitative evaluation by Vlachos, Korhonen & Ghahramani
(2009): we represented each clustering sample as a linking matrix and used the links that
occur in a proportion greater than a given threshold problink of the samples to define the
final clustering.
As an illustrative example, let us consider three clustering samples Si of four elements xi :
S1 = {x1 , x2 }, {x3 , x4 }
S2 = {x1 , x3 , x4 }, {x2 }
S3 = {x1 , x4 }, {x2 , x3 }
A clustering sample S can be represented by a linking matrix M = (mij ), where
(
1, if xi and xj belong to the same cluster; and
mij =
0, otherwise.
The matrix M is symmetric by definition, and the elements mii do not contribute any useful information; we shall therefore represent it as an upper triangular matrix with dotted
diagonal. This gives the following matrices:






· 1 0 0
· 0 1 1
· 0 0 1









· 0 0 
· 0 0 
· 1 0 






M1 = 
M2 = 
M3 = 
· 1 
· 1 
· 0 






·
·
·
Averaging over the linking matrices is trivial:


· 1 1 2


1
1X
· 1 0 


Mi = 
M̄ =
3 i
3
· 2 

·
The final clustering S̄ is created by assigning all the pairs of elements xi and xj to the same
cluster if the corresponding m̄ij exceeds a threshold problink. Assuming that problink is set
to ½ in our example, m1,4 and m3,4 exceed the threshold, which gives rise to the following
clustering:
S̄ = {x1 , x3 , x4 }
The unified clustering thus obtained is partial, since instances which are clustered inconsistently in the set of samples will not be included. A lower problink threshold will in
general give larger, less homogeneous clusters and increased coverage because of additional
links between less similar instances. Note that this can either increase the number of clusters
when instances were not linked otherwise, or decrease it when linking instances that already
belong to other clusters.
96
chapter 6. replacement errors
Triples
In the clc
36,447
Found in the bnc
Present in cluster input
In clusters (problink 0.75)
—
(problink 0.85)
—
(problink 0.99)
24,993
Verbs
36,447
68.6%
36,413
36,370
32,809
32,229
29,769
Nouns
36,447
99.9%
99.8%
90.0%
88.4%
81.7%
35,672
34,204
31,503
29,337
23,963
97.9%
93.8%
86.4%
80.5%
65.7%
Figure 6.4. This table shows how many of the 36,447 triples extracted from the clc can be found as complete
triples in the bnc, as well as how many of them contain a verb/noun which can be found in one or more bnc
triples, and furthermore how this verb/noun coverage changes as a result of filtering and clustering for different
values of problink.
WordNet
WordNet 1st sense
problink 0.75
problink 0.85
problink 0.99
Verb clusters
Size Wrong
Noun clusters
Size Wrong
29.2
4.6
13.2
10.2
6.1
13.5
4.3
37.8
21.7
8.7
115
21
4
4
2
49
14
42
20
13
Figure 6.5. Average cluster size and number of words clustered together which should not have been. Results
are shown for different values of problink, and clusters constructed from WordNet synonyms are included for
comparison.
6.4.4. Evaluation
As we have seen, the verb and noun clusters consist of words taken from the bnc, whereas
we used data from the clc for testing. Fig. 6.4 shows that almost all the verbs that appear
in correct sentences in the clc can be found in the bnc, and very few of them are filtered
out before clustering because of low frequency; for nouns, the match between the clc
and the bnc is not quite as good, and a certain number of nouns get filtered out. Most
of the shortfall, however, is caused by words which do not appear in the final clustering
despite being given as input to the clustering algorithm, and this problem cannot be avoided
completely since there are likely to be some words whose prepositional preferences do not
correspond to any others’ and which therefore cannot be clustered without loss in precision.
It is also worth noticing that almost one third of the (v, p, n) triples from the clc are not
found in the bnc, despite good coverage of both verbs and nouns separately.
As mentioned in Sect. 6.4.2, we extracted from the clc a set of 2,100 pairs of verbs and
16,330 pairs of nouns which should not be clustered together, and we checked whether
or not the words in each pair are assigned to different clusters for a given clustering. The
results are summarised in Fig. 6.5 and show that our clusters do well according to this
measure, whereas WordNet synonym sets violate the criterion more often by up to an
order of magnitude for similar average cluster size.
For the rest of this chapter, results will be reported using the clusters we get with problink
6.5. experiments using the clusters
97
0.85, chosen as a compromise of coverage, cluster size and accuracy. This clustering contains 287 verb clusters of size 2–85, and 255 noun clusters of size 2–233. Samples from
some of the clusters are shown below:
Noun clusters
abolition, breeding, confirmation, erection, example, ...
academic, accountant, activist, admirer, american, pope, ...
act, charter, command, directive, grammar, law, ...
activity, agriculture, analysis, broadcast, census, ...
administration, bureau, bureaucracy, clan, company, ...
Africa, America, Anglia, Asia, Australia, Ayrshire, Baghdad, ...
aisle, alley, alleyway, corridor, glen, lane, meadow, ...
altitude, level.
apartment, ballroom, bar, barracks, bookshop, booth, ...
aquarium, ashtray, beirut, belfast, cemetery, gaol, oven, ...
contrary, grip, nuisance, toilet.
counter, desk.
discrimination, disruption, interference, loss.
Verb clusters
abandon, advertise, announce, revise, stipulate, stress, use.
abduct, actuate, administer, appropriate, back, betray, ...
abolish, cite, consider, diagnose, discover, doubt, enclose, ...
absorb, categorise, classify, infiltrate.
accuse, acquit, compose, conceive, convict, despair ...
achieve, attain, extinguish, manage, obey, reckon.
acquaint, associate, coincide, comply, cope, familiarise, ...
address, convey, explain, whittle.
adjourn, award, fine, offer, postpone, promise, refuse, ...
adjust, apply, suit, tailor.
alarm, appal, astonish, astound, dismay, distress, horrify, ...
annoy, bother, please.
ask, pity.
6.5. Experiments using the clusters
6.5.1. Preposition guessing
The task of choosing the/an appropriate preposition in a given context has been studied a
bit more than that of preposition correction and is useful to look at for evaluation purposes,
as well as to gain a better understanding of the issues involved. Given (v, p, n), where p is the
original, correct preposition, we let the system choose the ‘best’ or most likely preposition
p∗ based on v and n only, i.e.,
p∗ = argmax ν(v, p , n).
p
98
chapter 6. replacement errors
If p∗ = p, the correct preposition has been chosen. Of course, several prepositions may be
equally correct, potentially with different meanings, but this complication is typically not
dealt with directly. Rather, human performance on the same task without further access to
the context, or inter-annotator agreement, is used to establish an upper bound.
To avoid forcing a decision to be made when there is little evidence for any preposition, the
system is allowed not to suggest a preposition:
∗
p =„
argmax ν(v, p , n), if
p
none,
max
ν(v, p , n) > ν0 ;
p
otherwise.
Fig. 6.6 shows the results for different values of ν0 , and also when individual words v or
n are replaced by the corresponding clusters V or N. As can be seen from the graph, performance decreases when cluster-based counts are used instead of those based on individual
words. There is a bias against the clusters, however, since a word which does not belong
to any cluster prevents the correct preposition from being chosen. When only words that
appear in the final clustering are taken into account, as shown in Fig. 6.7, not only does
the difference diminish, but verb clusters actually outperform individual words. The fact
that clusters can replace individual words without a large decrease in precision on this task
lends support to the idea that prepositional choice is not completely idiosyncratic, as it
is clearly possible to group words together. The best model could well be one where highfrequency items are kept separate, whereas medium-frequency and low-frequency items can
be clustered together. An analysis of which words are clustered and which are not would
be a useful extension to this research.
Counts based on individual words and on clusters can also be combined: using clusters as
back-off would give something like the following:


ν(v, p , n), if max
ν(v, p , n) > ν1 ;
 argmax

p

p




argmax ν(V, p , n), otherwise, if max
ν(V, p , n) > ν2 ;


p

p

p∗ =
argmax ν(v, p , N), otherwise, if max
ν(v, p , N) > ν3 ;

p

p




argmax
ν(V, p , N), otherwise, if max
ν(V, p , N) > ν4 ;


p

p



none,
otherwise.
A problem with this particular formulation is that it is difficult to set the thresholds νi ; the
only clear intution is that ν1 , which corresponds to individual words, should probably be
set to a lower value than for instance ν2 , which corresponds to accumulated counts over
verb clusters, but the ideal value would then depend on the cluster size. The identity
ν(v, p, n)
argmax ν(v, p, n) = argmax P
= argmax π(p|v, n)
ν(v, p, n)
p
p
p
p
6.5. experiments using the clusters
99
P
1
0.8
R
0.6
0
0.2
0.4
0.6
Figure 6.6. Precision–recall curve for preposition guessing, choosing the most frequent preposition based on
individual words (continuous line), verb clusters (dotted), noun clusters (dashed) and both (dotted/dashed).
The test set consists of originally correct (not corrected) instances of prepositions from the clc.
shows that the most likely preposition p∗ in a given context can be expressed equivalently
in terms of probabilities π in place of raw counts ν. The back-off approach can then be
defined as


argmax π(p |v, n), if max
π(p |v, n) > π1 ;


p

p




argmax
π(p |V, n), otherwise, if max
π(p |V, n) > π2 ;


p

p

p∗ =
argmax π(p |v, N), otherwise, if max
π(p |v, N) > π3 ;

p

p




argmax π(p |V, N), otherwise, if max
π(p |V, N) > π4 ;


p

p



none,
otherwise.
Unlike the unbounded thresholds νi , the thresholds πi are probabilities in the range [0, 1]
and can, as a first approximation, be kept equal (π1 = π2 = π3 = π4 ) and adjusted en
bloc for different precision/recall trade-offs.
An alternative approach is to combine the individual and cluster probabilities to a single
100
chapter 6. replacement errors
P
1
0.8
R
0.6
0
0.2
0.4
0.6
Figure 6.7. Same experiment as in Fig. 6.6, but the triples which contain either a verb or a noun that has not
been clustered have been filtered out.
value,
π(p|v ∈ V, n ∈ N) = λ1 π(p|v, n) + λ2 π(p|V, n) + λ3 π(p|v, N) + λ4 π(p|V, N),
with λi ≥ 0,
P
λi = 1, and define the best preposition as
i
p∗ = „
argmax π(p|v ∈ V, n ∈ N), if
p
none,
max
π(p|v ∈ V, n ∈ N) > π0 ;
p
otherwise.
This allows the decision to be based on a combination of word-specific and more general
selection criteria, but at the same time requires evidence to be available at different levels
(except perhaps in the case of a low threshold π0 ). Setting the coefficients λi to equal values
(λ1 = λ2 = λ3 = λ4 = 1/4) is a reasonable starting point and gives good results compared
to more carefully chosen values (see Fig. 6.8).
6.5. experiments using the clusters
101
λ1
λ2
λ3
λ4
Correct
Incorrect
Accuracy
1
0
1/17
1/2
1/3
0
1/17
1/2
1/3
0
0
0
1/19
1/4
2/7
1/4
0
1
16/17
1/2
2/3
0
0
0
0
0
0
0
1/19
1/4
0
1/8
0
0
0
0
0
1
16/17
1/2
2/3
0
1/17
1/2
1/19
1/4
1/7
1/8
0
0
0
0
0
0
0
0
0
1
16/17
1/2
16/19
1/4
4/7
1/2
34,058
38,320
38,547
38,738
38,780
40,948
41,335
41,920
41,963
42,574
42,846
43,397
43,491
44,439
44,525
44,615
50,189
45,927
45,700
45,509
45,467
43,299
42,912
42,327
42,284
41,673
41,401
40,850
40,756
39,808
39,722
39,632
40.4%
45.5%
45.8%
46.0%
46.0%
48.6%
49.1%
49.8%
49.8%
50.5%
50.9%
51.5%
51.6%
52.7%
52.9%
53.0%
Figure 6.8. Performance on the task of preposition guessing evaluated on a development set extracted from the
bnc for different values of the coefficients λi . ‘Correct’ means that the preposition found to be most probable
is the one that was actually used in the bnc; ‘incorrect’ means that it is not the one actually used in the bnc
(although there are often more than one possibility, so the preposition chosen by the system is not necessarily
unreasonable). Each of the coefficients λ1 , λ2 , λ3 and λ4 was set to ¼, ½, 1, 2 and 4 (normalised in the
table above); the table shows a small number of illustrative results, sorted according to performance. The best
performance, 53.0% accuracy, is obtained for a particular set of non-zero values of λi , compared to 52.7% for
equal values of λi = 1/4. Without clustering, the result is 40.4% accuracy, whereas using the clusters in lieu of
individual counts gives an accuracy of 50.5%.
6.5.2. Detecting preposition errors
According to Izumi & al., the preposition error rate for learners of English can be as high
as 10 per cent (2003); in other words, every tenth preposition is wrong. Later publications
on automatic error detection have reported performance on test sets with equally high error
rates, and we chose to use a test set with 10 per cent error rate as well in order to obtain
comparable results at least in that respect.
Our test set consists of about 1,500 incorrect instances extracted from the clc, as well as
nine times as many instances which are not marked up as erroneous and therefore taken
to be correct. The (v, p, n) triples are all extracted from phrases where the preposition
immediately follows the verb to minimise the possibility of syntactic misanalysis.
It is important to realise that preposition guessing and error detection are two distinct problems, and that a system with good performance on the former is not on its own sufficient for
the latter task. For the sake of example, let us assume that we have at our disposal a system
which is able to predict the correct preposition 70 per cent of the time, and that we want to
detect incorrect prepositions in a text where 10 per cent of the prepositions are wrong. One
might consider the original preposition to be incorrect when the system chooses a different
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chapter 6. replacement errors
preposition, which would give the following confusion matrix (assuming that there is only
one correct preposition in a given context):
Correct
Incorrect
Σ
Classified ‘correct’
Classified ‘incorrect’
0.63
0.27
0.03 − ε
0.07 + ε
0.66 − ε
0.34 + ε
Σ
0.90
0.10
1
The proportion ε represents the cases where the system has predicted an incorrect preposition distinct from the original one. As can be seen, the over-all precision may be as low
as 0.07/0.34 or 21 per cent, which means that the system will give four false positives for
each incorrect preposition detected, making it practically unusable. In general, precision is
considered to be more important than recall for this task.
One way of achieving higher precision is to consider a preposition q as incorrect only in the
case of there being ample evidence for another preposition in the same context, for instance
if there exists a preposition p∗ such that
P
p
ν(v, q, n) + 1
ν(v, p∗ , n)
>P
+ π0 ,
ν(v, p, n) + 1
ν(v, p, n) + 1
p
where one occurrence of the instance under consideration (v, q, n) has been added to the
bnc-based counts ν in order to deal more gracefully with instances not observed in the bnc.
To avoid confusion with previous notation, π̄ will be used for thus modified probabilities
based on bnc counts, which enables us to rewrite the inequality as
π̄(p∗ |v, n) > π̄(q|v, n) + π0 .
The dotted line in Fig. 6.9 shows precision and recall for this method.
Both approaches for making use of the clusters as described earlier in the case of preposition
guessing (viz, fall-back and smoothing) can be adapted to work for error detection as well.
In the first case, q will be considered incorrect if at least one of the following relations
holds:
π̄(p∗ |v, n) > π̄(q|v, n) + π1
π̄(p∗ |V, n) > π̄(q|V, n) + π2
π̄(p∗ |v, N) > π̄(q|v, N) + π3
π̄(p∗ |V, N) > π̄(q|V, N) + π4
In the second case, q will be considered incorrect if
π̄(p∗ |v ∈ V, n ∈ N) > π̄(q|v ∈ V, n ∈ N) + π0 ,
6.5. experiments using the clusters
103
P
1
0.8
0.6
0.4
0.2
R
0
0.2
0.4
0.6
0.8
Figure 6.9. Precision–recall curve for error detection without clustering (dotted), with cluster fall-back
(dashed), and with cluster smoothing (continuous line).
where
π̄(p|v ∈ V, n ∈ N) = λ1 π̄(p|v, n) + λ2 π̄(p|V, n) + λ3 π̄(p|v, N) + λ4 π̄(p|V, N).
Fig. 6.9 shows the results for both methods with λ1 = λ2 = λ3 = λ4 = 1/4 and for
different values of the parameter π1 = π2 = π3 = π4 in the first case and π0 in the second
case.
Both methods give a noticeable increase in performance, roughly a five percentage points’
increase in precision for a given recall value over large parts of the precision–recall curve,
though admittedly neither method consistently outperforms the baseline (no clusters) for
all recall points. This shows that verb/noun clustering can provide increased performance
in the task of detecting prepositional choice errors, and we hope that further enhancements,
such as training the parameters λi and πi or finding a way to combine the two methods, can
provide further improvements.
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chapter 6. replacement errors
6.6. Discussion and future work
The smoothing method in particular allows a significant increase in recall at high precision;
with 15.4 per cent recall at 82.7 per cent precision, the numbers actually outperform the
state of the art as exemplified by Tetreault & Chodorow’s 14.1 per cent recall at 82.1
per cent precision (2008). These results are based on different test sets and thus not directly
comparable, but it is still interesting to have a look at methodological differences. Tetreault & Chodorow’s test set presumably contains all kinds of prepositional choice errors,
whereas ours is restricted to prepositions with a verbal head and a nominal dependent, a
subset for which the choice of preposition can be expected to be more constrained, but
which also excludes a number of errors that are both frequent and trivial to detect (e.g., the
adverbial *on/at Tuesday), thus potentially contributing to high precision and reasonable
recall. As features, they used the head of the preceding verb phrase and following noun
phrase, approximating the verb and noun features used in our model, but also added two
words on either side of the preposition and their parts of speech as features, along with
a score of others; furthermore, heuristic rules were added, such as the one excluding antonyms as potential corrections, or the one preventing the system from ever considering ‘for
+ human noun’ as incorrect given that it can allegedly occur quite freely in contexts where a
different preposition might be expected, but would change the meaning completely; finally,
the problem of detecting extraneous prepositions was added in, which seems likely to have
caused an increase in performance given a large proportion of trivial errors like many *of
people and friends with *with, though this is difficult to ascertain from the paper. To give
two concrete examples from the clc that may be representative of frequent trivial errors
in other learner corpora as well, consider that as many as 1.2 per cent of the extraneous
prepositions in single-error sentences occur as part of the phrase to (tele)phone *to the,
and that over 0.7 per cent of the prepositional choice errors involve the expression to look
forward to. Additional features and heuristic rules would seem to be complementary to
clustering techniques such as the ones presented in this paper, and combining the two is an
area for further research.
As for the clustering itself, using the prepositions as features for clustering verbs and nouns
independently has been shown to improve the performance in the tasks of preposition correction and guessing, but there is clearly a potential for better clustering based on additional
features. Verbs and nouns interact via the preposition, which is not captured by the two
independent clustering models used in our work. It would be possible to supplement the
prepositional features used for verb clustering with the nouns and vice versa, but such a
process would result in an extremely sparse feature set. Clustering the verbs first and using
the verb clusters instead to refine the prepositional features used for noun clustering would
cause asymmetry between the noun and verb clustering, and the same issue would of course
apply if it were done the other way round. A more elegant solution would be a bi-clustering
approach, such as the infinite relational model (Kemp & al. 2006).
Chapter 7.
Semi-automatic
annotation
anual error annotation of learner corpora is time-consuming and error-prone,
whereas existing automatic techniques cannot reliably detect and correct all types
of error. In this chapter, we shall investigate the feasibility of integrating automatic methods
such as the ones described in previous chapters into the annotation process and thereby
obtaining annotation of higher quality with less manual effort.
M
7.1. Status quo
The number of sizeable error-annotated corpora remains limited, at least partly because ‘error annotation is one of the most tedious, subjective, time-consuming and labo[u]r-intensive
aspects of corpus annotation’ (Wible & al. 2001). The clc is, with over 16½ million
words error-coded, one of the largest corpora of this kind currently in existence. The initial
error annotation is done manually by a small team of trained annotators who type sgml
tags and proposed corrections using a standard text editor, followed by a post-editing step
designed to detect not only occasional sgml errors, but also inconsistencies in the annotation (referred to as ‘detoxification’ by the error coders). A fairly pragmatic and linguistically
superficial set of error tags ‘designed in such a way as to overcome [ . . . ] problems with
the indeterminacy of some error types’ (Nicholls 2003) and a reasonably thorough coding manual further contribute to increasing the annotation quality. The error annotation
was originally performed by one person; with the expansion of the team, this person no
longer does any of the initial annotation, but instead reads through all the annotated scripts
to assure a high level of consistency. Nevertheless, we are not aware of any formal interannotator agreement studies having been performed, and it is of course difficult to obtain
106
chapter 7. semi-automatic annotation
a quantitative measure of how good the annotation actually is. It should also be noted that
certain errors are not marked up as such in the clc, in particular misspelt proper names
(apart from the most well-known ones, e.g., *Ingland and *Amrica) and errors which appear to be directly induced by the exam question; this is a sensible compromise seen from a
textbook writer’s perspective, but it inevitably causes certain classes of incontestable error
not to be marked up and thus to remain indistinguishable from correct text, which makes
the corpus less suitable for other applications. Certain other practices are suboptimal because they make it difficult to tell what exactly was part of the original examination script
and what has been modified later, for instance the somewhat crude anonymisation technique that consists in substituting a number of x’s for proper names and other potentially
sensitive information; given the types of information that is typically removed by this means,
simply considering x’s as a noun phrase often works in practice, though.
To get an idea of the level of consistency in some fairly clear-cut cases, we identified words
and phrases often marked up as erroneous either unconditionally or in given linear contexts
and checked whether remaining identical occurrences actually should have been marked up
as well. As Fig. 7.1 shows, the level of consistency is generally high for obvious errors.
However, a trivial error like occured spelt with one r has actually been missed 15 per cent
of the times it occurs, which seems to indicate that a hybrid system where simple errors
are marked up automatically could usefully complement the human annotator by spotting,
in particular, typographical errors and others that are easily overlooked. Furthermore, it
turns out that certain trivial errors are inordinately frequent (cannot incorrectly split into
can not alone accounts for 0.2 per cent of the errors), which implies that even a relatively
crude system would be able to deal with a meaningful subset of the errors and let the
human annotator concentrate on more interesting/complex ones. As for more subtle details,
upon which style guides are likely to disagree, the lower consistency rates arguably indicate
that the corresponding putative rules are not universally followed; whether or not Third
World should be capitalised is purely conventional, and there is no obvious reason for
requiring a comma in on the other hand, I want to improve my conditions of employment,
but not in on the other hand I agree with the complaints, though it is difficult to tell
from the corpus whether the annotators have attempted always to require a comma after
sentence-initial adverbials and sometimes forgotten, or only require it when its absence
would cause ambiguity, at least locally (‘garden paths’), and sometimes added commas
which are not strictly necessary. Another possible explanation could be that the annotators
try to render the use of commas consistent within each script rather than globally, though
such an approach seems unlikely to result in high correction rates for specific expressions;
or the policy could have changed as the consequence of a shift in usage observed amongst
professional writers and publishers (this was reportedly the case for hyphens in attributive
compounds, which are no longer considered compulsory in general). Whatever the cause
might be, inconsistencies make the corpus less suitable as training data or as a gold standard
of errors for an automatic system to detect; such issues are to a certain extent inherent in the
7.2. automatic pre-annotation
107
Original
Correction
accomodation
a lots of
forward to hear
Your faithfully
appreciate if
to spent
center
However there
a part time
occured
On one hand
third world
other hand I
accommodation
a lot of
forward to hearing
Yours faithfully
appreciate it if
to spend
centre
However, there
a part-time
occurred
On the one hand
Third World
other hand, I
Rate
99%
99%
99%
>96%
96%
95%
91%
87%
86%
85%
60%
57%
∼50%
Figure 7.1. Correction rate, the proportion of incorrect occurrences of a word/phrase that are actually marked
up. (Some of the words/phrases that should typically be corrected may be correct in specific cases; such
instances were excluded before the rates were calculated.)
task of error annotation, but it is to be hoped that more sophisticated consistency checks
can contribute to detect current inconsistencies, leading the way to clearer guidelines, at
least some of which can be enforced mechanically.
7.2. Automatic pre-annotation
Despite considerable work on methods and systems to detect and correct spelling and grammar errors, none of the existing error-annotated corpora seem to have been prepared using
such techniques. In order to investigate the potential of semi-automatic annotation in terms
of making the human annotator’s task less laborious and repetitive, a system was developed
that aims to detect trivial or frequent errors automatically and add the corresponding annotation, including corrections when appropriate (this section) and combined with an error
annotation tool to make the error annotation more readable for the annotator and easier
to manipulate (next section).
7.2.1. Purveyors of perpetual perplexity
Many trivial errors are committed — and corrected — over and over again, such as the ones
shown in Fig. 7.1. For the purposes of this experiment, rules for automatic correction of
common errors were derived directly from the existing error annotation: a correction rule
was created for errors that appear at least 5 times and are corrected in the same way at least
90 per cent of the time. In addition to the text marked up as erroneous, up to one word on
either side was used to model the immediate context in which an error occurs. For instance,
the correction I <sx>thing|think</sx> that would give rise to four potential indicators of
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chapter 7. semi-automatic annotation
error, thing, I thing, thing that and I thing that, each of which can be searched for and
counted in the corrected text; in this case, the result of such an investigation would be that
at least I thing that is non-existent or extremely rare in the corrected text and thus a good
indicator of error, and furthermore that the error is always or most of the time corrected
in the same way (i.e., to I think that); the conclusion would be that an automatic system
ought to hypothesise every occurrence of I thing that as a misspelling of I think that.
The following examples illustrate the kinds of error that can be detected and corrected using
such simple rules; the previously discussed example appears as I <sx>thing|think</sx> that,
which means that any occurrence of I thing that will result in thing being marked up as a
spelling confusion (sx) error for think.
No context:
<s>accomodation|accommodation</s>
<sa>center|centre</sa>
<rp>french|French</rp>
<rp>an other|another</rp>
<up>I’am|I am</up>
<mp>above mentioned|above-mentioned</mp>
<w>be also|also be</w>
<id>In the other hand|On the other hand</id>
Left context:
the <rp>internet|Internet</rp>
reason <rt>of|for</rt>
all <agn>kind|kinds</agn>
I <sx>though|thought</sx>
despite <ut>of|</ut>
computer <rn>programme|program</rn>
to <dv>complaint|complain</dv>
Right context:
<da>Your|Yours</da> Sincerely
<agd>this|these</agd> things
<mp>long distance|long-distance</mp> travel
<uv>be|</uv> appreciate
<rv>loose|lose</rv> their
Left and right context:
50 <mp>years|years’</mp> experience
I <sx>thing|think</sx> that
I <dv>advice|advise</dv> you
a <dj>slightly|slight</dj> increase
is <sx>to|too</sx> small
As we have seen, certain errors are not always corrected even if they should have been, so
requiring 100 per cent correction rates would hardly induce any rules at all. Furthermore,
a rule can be useful even if it occasionally ‘corrects’ what was correct in the first place since
it is less arduous for the human annotator to remove incorrect error mark-up from time
7.2. automatic pre-annotation
109
to time than to add it in the common case (e.g., a rule that indiscriminatively annotates
can not as a misspelling of cannot will fail in cases like can not only ..., but also, but this
is of little consequence since incorrect instances of can not in the clc outnumber correct
ones by almost two orders of magnitude). One should keep in mind, though, that human
annotators reportedly find spurious errors introduced automatically particularly annoying,
so an imperfect rule should only be added when the resulting annotation is correct in an
overwhelming majority of the cases; actually, 90 per cent is probably far too low from
that perspective, but the previous considerations of annotation consistency make it seem
likely that errors which have been corrected by the annotators 90 per cent of the time really
should be corrected more often, so we expect the rules to be somewhat more reliable than
the threshold suggests. Unfortunately, the threshold chosen precludes some obvious errors
from being identified (e.g., *occured), but then a lower threshold could easily lead to too
many spurious errors for the human annotator to remove. (Some of these errors will in
any case be identified by other methods, as described in the following sections.) Manual
evaluation of specific rules might be worthwhile if such a system is to be employed on a
large-scale annotation project, but would clearly require a fair amount of work by someone
who can make policy decisions on what should and should not be marked up as erroneous
and was not feasible within the scope of this study.
7.2.2. Morphological metamorphosis
The corpus-derived rules described in the previous section work well for specific words
which are both frequent and frequently misspelt in the clc, but do not generalise to similar
or even virtually identical errors involving different words. Travel and tourism seem to be
a popular topic in Cambridge exams, so the misspelling of travelled as *traveled with one
l is amply exemplified, whereas *signaled occur only once, so no corresponding correction
rule will be generated when using the proposed method and thresholds, and no rule can
be derived from the corpus for a word like *groveled, which does not occur at all, but
might well appear in the future; these errors all have to do with British English rules for
l-doubling in morphological derivatives, and they can therefore be handled systematically,
provided we have access to a word’s correct morphology.
Without trying to make the corrected exam scripts conform in all respects to cup’s house
style, the clc annotators naturally use Cambridge dictionaries to settle any doubts regarding orthography and morphology, albeit reluctantly in cases where the last editions do not
yet reflect what is about to become established usage. It would therefore be preferable
to use a Cambridge dictionary as the basis for automatic annotation rules; unfortunately,
though, the ones available to us do not contain sufficient information on inflectional morphology, so we had to use a different data source and chose the Lexical Database developed
by the Dutch Centre for Lexical Information (celex), which in addition contains useful information about noun countability and derivational morphology.
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chapter 7. semi-automatic annotation
The examples below illustrate the types of error that can be automatically detected and corrected by predicting systematic morphological anomalies modelled on actual errors found
in the clc.
Plural of singulare tantum:
<cn>abhorrences|abhorrence</cn>
<cn>bigamies|bigamy</cn>
<cn>blamelessnesses|blamelessness</cn>
Derivation of adjective:
<dj>academical|academic</dj>
<dj>atypic|atypical</dj>
<dj>cheerfull|cheerful</dj>
<dj>non-legal|illegal</dj>
<dj>inlegible|illegible</dj>
<dj>unmature|immature</dj>
<dj>inpossible|impossible</dj>
<dj>inrational|irrational</dj>
<dj>uncommissioned|non-commissioned</dj>
<dj>incertain|uncertain</dj>
Derivation of adverb:
<dy>abnormaly|abnormally</dy>
<dy>academicly|academically</dy>
<dy>accidently|accidentally</dy>
<dy>accuratly|accurately</dy>
<dy>angryly|angrily</dy>
<dy>barily|barely<dy>
<dy>closelly|closely</dy>
<dy>wishfuly|wishfully</dy>
Adjective inflection:
<ij>biger|bigger</ij>
<ij>brainyer|brainier</ij>
<ij>crazyest|craziest</ij>
<ij>grimest|grimmest</ij>
<ij>Chineses|Chinese</ij>
Noun inflection:
<in>addendas|addenda</in>
<in>addendums|addenda</in>
<in>alumnas|alumnæ</in>
<in>amanuensises|amanuenses</in>
<in>anthologys|anthologies</in>
<in>antiheros|antiheroes</in>
<in>bagsfuls|bagsful</in>
<in>boleroes|boleros</in>
<in>nucleuses|nuclei</in>
<in>oxes|oxen</in>
<in>paterfamiliases|patresfamilias</in>
<in>persona non gratas|personæ non gratæ</in>
<in>schemas|schemata</in>
7.2. automatic pre-annotation
111
<in>tooths|teeth</in>
<in>aircrafts|aircraft</in>
Verb inflection:
<iv>abandonning|abandoning</iv>
<iv>abbreviateing|abbreviating</iv>
<iv>abhoring|abhorring</iv>
<iv>abolishs|abolishes</iv>
<iv>accompanys|accompanies</iv>
<iv>amplifis|amplifies</iv>
<iv>abolishd|abolished</iv>
<iv>abolisht|abolished</iv>
<iv>abstainned|abstained</iv>
<iv>accompanyed|accompanied</iv>
<iv>ferrid|ferried</iv>
<iv>airdroped|airdropped</iv>
<iv>breeded|bred</iv>
<iv>slidden|slid</iv>
7.2.3. Spell-catching
The celex database distinguishes between British and American spellings, so a list of American words which do not exist in British English can be derived as well:
<sa>britches|breeches</sa>
<sa>jewelry|jewellery</sa>
<sa>maneuver|manœuvre</sa>
<sa>Cesarean|Cæsarean</sa>
Proper names and other words always written with a capital letter were extracted from the
database to deal with capitalisation errors:
<rp>gouda|Gouda</rp>
<rp>teutonic|Teutonic</rp>
<rp>euclid|Euclid</rp>
<rp>scotland|Scotland</rp>
<rp>christmastime|Christmastime</rp>
Finally, a list of correct word forms was extracted from the database to enable detection
of mundane spelling errors: words consisting entirely of lowercase letters not already corrected in one of the previous steps and not in the wordlist can be identified as likely typographical errors. For such words, the correct spelling is unknown, and a distinguished
token (¿?) takes the place of a correction to indicate this. It would of course be possible to
make the system propose a plausible correction, for instance by using methods like the ones
proposed by Deorowicz & Ciura to model the kinds of errors typically committed (2005),
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chapter 7. semi-automatic annotation
relying on statistics from the clc for error frequencies; this would clearly be useful in a
tool aimed at less confident language users and would be an interesting extension to the
system, but seems less important in the context of error annotation and is unlikely to have
a significant impact on the annotation speed given that all frequent errors with obvious
corrections will have been handled by the corpus-derived rules. Misspelt words containing
at least one capital letter are not detected, partly because we have not tried to compile a
comprehensive lexicon of names, partly because of the clc policy of not correcting proper
names in general.
7.2.4. Euphonia
We have previously (see Sect. 4.4) obtained good results with machine-learning techniques
on the task of detecting incorrect choice between the two euphonic variants of the indefinite
article (a/an). The rule is to use a before a consonant and an before a vowel; the celex
database provides pronunciations and we used the database for other error types already,
so it seemed natural to take advantage of that information. Only the first sound in a word
is significant for the choice of a or an, and only whether it is consonantal or vocalic, as
illustrated in the following examples:
minister consonant
MP vowel
open vowel
one consonant
home consonant
hour vowel
hotel vowel/consonant
utter vowel
useful consonant
Uruguay vowel/consonant
Words with alternative pronunciations, such as Uruguay, may be used with either form of
the article. (The traditional usage of an in front of unaccented aspirated h is not accounted
for, but the information needed is available in the celex database, so this could easily be
added.) The text is part-of-speech-tagged with rasp (Briscoe, Carroll & Watson 2006)
before this step to avoid conflation of unrelated occurrences of a with the indefinite article.
7.2.5. Synopsis
The flowchart in Fig. 7.2 illustrates how the different parts of the system interact to produce automatic annotation. The contents of all the intermediate files produced from a
short example file can be found in Appendix d.3. Each exam script in the clc contains
information about the candidate and the exam taken, as well as the actual text written:
parseclc.pl
file.0
file
file.2
file.3
tokenise
tag
file.2tok
file.3pos
ann0
ann2
ann3
file.0tag
file.2tag
file.3tag
weave & split
folder ”
file1
..
.
filen
Figure 7.2. Schematic overview of the automatic annotation process, starting with a single file containing
multiple unannotated exam scripts and ending up with a set of files, each containing an annotated script.
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chapter 7. semi-automatic annotation
<candidate>[metadata about examination and examinee]</candidate>
<text>
<answer1><question number>9</question number> [...] <original answer>
<p>I wont be going to the little nice persian caf&eacute; this after noon becauce I eated to much
for lunch and have now a awfull stomachache.</p>
</original answer></answer1>
[more answers]
</text>
parseclc extracts the text to analyse:
<p>I wont be going to the little nice persian café this after noon becauce I eated to much for
lunch and have now a awfull stomachache.</p>
— file.0
The only detail worth mentioning at this point is the normalisation of &eacute; to é; xml
provides many ways to represent a given character, and ensuring that é never appears as
for instance &eacute; or &#xE9; simplifies further processing. The first error tags are then
added by ann0, using simple string matching:
<p>I <e t="MP"><i>wont</i><c>won’t</c></e> be going to the
<e t="W"><i>little nice</i><c>nice little</c></e> persian café this
<e t="RP"><i>after noon</i><c>afternoon</c></e>
<e t="S"><i>becauce</i><c>because</c></e> I eated
<e t="SX"><i>to</i><c>too</c></e> much for lunch and
<e t="W"><i>have now</i><c>now have</c></e> a
<e t="DJ"><i>awfull</i><c>awful</c></e>
<e t="RP"><i>stomachache</i><c>stomach ache</c></e>.</p>
— file.0tag
At this point, error tags have been added, which will have to be removed before the next
processing step. parseclc again extracts the text, using the corrections rather than the
original text when applicable:
<p>I won’t be going to the nice little persian café this afternoon because I eated too much for
lunch and now have a awful stomach ache.</p>
— file.2
This partly corrected version of the text is then passed through rasp’s sentence splitter
and tokeniser. ann2 detects further errors based on the tokens and adds annotation. The
process is repeated once more, this time adding part-of-speech tags for ann3 to work on,
which gives the following annotated output:
<p>I <e t="MP"><i>wont</i><c>won’t</c></e> be going to the
<e t="W"><i>little nice</i><c>nice little</c></e>
7.3. annotation tool
115
<e t="RP"><i>persian</i><c>Persian</c></e> café this
<e t="RP"><i>after noon</i><c>afternoon</c></e>
<e t="S"><i>becauce</i><c>because</c></e> I
<e t="IV"><i>eated</i><c>ate</c></e>
<e t="SX"><i>to</i><c>too</c></e> much for lunch and
<e t="W"><i>have now</i><c>now have</c></e>
<e t="FD"><i>a</i><c>an</c></e> <e t="DJ"><i>awfull</i><c>awful</c></e>
<e t="RP"><i>stomachache</i><c>stomach ache</c></e>.</p>
— file.3tag
Because the xml mark-up is handled properly, the fact that awful is embedded within an
error tag does not prevent the system from detecting that the preceding determiner should
be an rather than a. If parseclc were applied again, the following output would have been
generated:
<p>I won’t be going to the nice little Persian café this afternoon because I ate too much for lunch
and now have an awful stomach ache.</p>
The process can obviously continue with, for instance, the generation of syntactic annotation as the basis of additional error annotators, such as the ones discussed in the previous
chapter. For the purposes of this experiment, though, the output from ann3 was combined
with the original file (containing metadata irrelevant for the automatic error annotation) to
create complete automatically annotated files for the human annotator to work on.
7.3. Annotation tool
Whereas some corpora have been annotated using dedicated tools such as the Université
Catholique de Louvain Error Editor (uclee, cf. Dagneaux, Denness & Granger 1998),
the clc annotators have written sgml tags directly in a text editor. This is not necessarily
an impediment to efficient annotation when compared to systems which require error tags
to be selected from menus and submenus given that the coding scheme uses short codes
and makes judicious use of sgml abbreviation techniques in order to limit the number
of characters and thus keystrokes needed to mark up an error. The code is also quite
readable as long as there are not too many nested errors, but occasional sgml errors, which
render the entire file in which they occur unparseable until the error has been corrected, are
nevertheless difficult to avoid completely.
An additional consideration for semi-automatic annotation is the ease with which an incorrect error tag added by the machine can be removed by the human.
We felt that a simple annotation tool was the right solution to these problems: it would
provide a graphical representation of the error annotation, making it easier for the annotator to see where each error begins and ends, in particular in the case of nested errors; the
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chapter 7. semi-automatic annotation
Figure 7.3. The pre-annotated example sentence as it appears in the annotation tool. For each error annotation, the error type is shown to the left, on an orange background; the error in the middle, on a red background;
and the correction to the right, on a green background.
number of keystrokes needed could be reduced further, and the need for typing ‘exotic’ characters eliminated; sgml errors would never appear; and one keystroke would be sufficient
to remove an unwanted error tag.
Fig. 7.3 shows how a sentence with error annotations appears in the annotation tool.
7.4. Annotation experiment
The head annotator of the clc kindly agreed to annotate text taken from previously unannotated parts of the corpus using the system described on the previous pages. After initial
testing and development, four different set-ups were tried, as described in the following
sections, in order to investigate the contribution of different factors.
7.4. annotation experiment
CLC coding
—— & detox
Part 1
Part 2
Part 3A
Part 3B
Part 3 (Σ)
Part 4
117
Words
Tags
Words/tag
Hours
Words/hour
Tags/hour
6,736,452
——
746,252
——
9
——
5,156
6,924
1,306
972
145
108
13,127
19,716
9,881
9,679
19,560
18,610
934
1,433
311
1,023
1,355
1,373
14
13.8
31.7
9.46
14.65
13.55
4
?4
1.51
2.71
4.22
1.66
3,281
4,929
6,544
3,572
4,635
11,210
233
358
206
377
316
827
Figure 7.4. Performance in terms of annotation speed. The first two lines of the table relate to the part of the
clc that has been error-coded during the last couple of years, the figures on the first line only including the
time spent on the initial coding, the second line including the subsequent post-editing step (‘detoxification’ to
remove sgml errors and coding inconsistencies) as well; the remaining lines relate to the annotation produced
as part of the annotation experiment described in this chapter, parts 1–4 being described in Sect. 7.4.1–7.4.4.
The number of words and tags is indicated for each part of the corpus, and the inverse tag density (words per
tag) is calculated to give an idea of the amount of errors (more words per tag means fewer errors, higher-quality
text and less work for the annotator). The number of hours spent to annotate (including post-editing in the
case of the second line) each part is indicated, which, in combination with word and tag counts mentioned
previously, allow the annotation speed to be calculated in terms of wordsper hour as well as tags per hour.
7.4.1. Manual annotation (part 1)
Statistics from previous years of clc annotation enable us to estimate average annotation
speed in terms of tags per hour or words per hour. We were concerned that those data points
might not be directly comparable with the ones obtained as part of the experiment, though,
and therefore included a batch of scripts for manual annotation, asking the annotator to
type tags in a text editor as previously.
As expected, a few sgml errors appeared:
Mismatched tags:
<#DK>competitable|competitive</#DJ>
<#RJ>fashion|fashionable</#DJ>
<#SA>humor|humour</#SX>
<#FV>making|to make</#RV>
Missing angle bracket:
<#DJ>successfull|successful</#DJ
Complex error:
<#UV>I’m</#I> (for <#UV>Im|I</#UV>)
There were also some overlooked29 errors which the automatic system would have detected:
<rp>clare|Clare</rp>
<rp>10’000|10,000</rp>
29
) The annotator later told that the first two errors were deliberately ignored.
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chapter 7. semi-automatic annotation
Part 2
Part 3A
Part 3B
Part 3 (Σ)
Part 4
Before
Correct
After
P
R
372
0
397
345
1,448
280
1,023
93%
24%
89%
99%
27%
94%
1,302
353
1,293
1,373
Figure 7.5. Performance in terms of precision and recall of errors by the pre-annotation system measured
against the human annotator. The before column indicates the number of tags added during the pre-annotation
step; the after column indicates the total number of errors after annotation; and the correct column indicates
the intersection between the two sets (i.e., the number of tags added during pre-annotation that were not
subsequently removed during annotation). Note that Part 4 uses human pre-annotation (resulting from the
Part 3 annotation).
<sa>analyze|analyse</sa>
<sa>analyzed|analysed</sa>
an <mp>all time|all-time</mp> low
our <mp>day to day|day-to-day</mp> life
it is <sx>to|too</sx> complicated
had to <rv>seat|sit</rv> in the back row
As for the annotation speeds in this experiment compared to previous annotation of the
clc, the two turned out to be significantly different (see Fig. 7.4); this can at least in part be
ascribed to better English with fewer errors in the experiment (on average 1 error tag added
per 14 words) than in previously annotated parts of the corpus (1 tag per 9 words), and
is thus not entirely surprising, but it also shows that any direct comparison with previous
years’ results is likely to be misleading.
7.4.2. Semi-automatic annotation (part 2)
For the second part of the experiment, scripts were pre-annotated automatically using the
system described in Sect. 7.2 before it was given to the human annotator for correction and
supplementation using the annotation tool. Examination of the final annotation showed
that the automatic pre-annotation system had precision of 93 per cent and recall of 24 per
cent (see Fig. 7.5), which is quite encouraging given that the system is neither comprehensive nor fine-tuned: increased recall without loss in precision can be obtained by extending
the system’s coverage, and precision is impeded by an incomplete and slightly outdated lexicon. The annotation speed turned out to be about 50 per cent higher than in the previous
experiment; in addition to this, there are no sgml errors to correct, and the annotation
is more consistent, which eliminates the need for subsequent sgml verification and vastly
reduces the need for consistency checking, thus making the effective speed increase closer
to 100 per cent.
7.5. conclusion
Part 3A
Part 3B
Part 3 (Σ)
119
Total
Before
686
517
0
297
Tagged sentences
Correct After
271
198
417
P
R
Classification
P
R
(71%)
91%
81%
44%
68%
Figure 7.6. Performance in terms of the system’s ability to detect sentences containing at least one error.
The total column shows the total number of sentences. The before, correct and after columns have the same
meaning as in Fig. 7.5, but they refer to a number of sentences rather than a number of individual errors.
7.4.3. Annotation of individual sentences classified as good/bad (part 3)
In order to get a better idea of how important context is for correct annotation, as well as
to assess the potential for more efficient annotation by focusing on sentences more prone to
contain errors, sentences were split into two sets, likely correct (3A) and likely containing
errors (3B), for the third part of the experiment. As one would expect, this set-up caused
the annotation speed in terms of words per hour to increase for the largely correct sentences,
and in terms of error tags per hour for the largely incorrect sentences, whilst both performance measures declined globally, at least partly because it is more burdensome and mentally
exhausting for the annotator to deal with individual sentences than connected passages of
discourse. Sentence-level performance is indicated in Fig. 7.6.
7.4.4. Re-evaluation in context (part 4)
Finally, the sentences from part 3 were put together again and presented to the annotator
anew for evaluation in context. This gave a precision figure for manual detection of errors
in individual sentences out of context of well over 99 per cent, whereas recall was a bit
lower at 94 per cent; the conclusion from this is that what appear to be errors when a sentence is regarded in isolation usually turn out to be errors in context as well, whereas about
1 out of 20 errors, including a high proportion of tense errors, require extra-sentential
information to be detected. This part of the experiment also permitted us to calculate an
upper bound for annotation speed given very high-quality pre-annotation: compared to
part 1, there was an increase of 250 per cent in annotation speed, or approaching 325
per cent if the amount of post-editing can be reduced, although the fact that the annotator
had already seen the sentences, albeit out of order, may also have contributed to the speed
increase observed in this part of the experiment..
7.5. Conclusion
We have seen that an annotation tool that incorporates automatic techniques for error
detection and correction can contribute to higher accuracy and increased productivity in
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chapter 7. semi-automatic annotation
the task of error annotation. This is significant since manual error annotation can be both
laborious and tedious, whereas the existence of sizeable error-annotated corpora is crucial
both for the study of language containing errors (be it from a pedagogic or a more purely
linguistic perspective) and for the development of ‘grammar checkers’ and other tools that
actually address the areas of language that can be shown to be problematic. Higher recall
can obviously be achieved by adding error detectors for further types of error, and the
techniques currently employed can also be refined to give higher performance.
Chapter 8.
Conclusion
s laid out in section 1.5, the main goal of this doctoral work has been to develop
methods for automatic detection and correction of grammatical errors.
A
Given that corpus material is used not only to validate the detection techniques, but also
in many instances constitutes the evidence upon which they rely to distinguish between
right and wrong, corpus encoding techniques and methods that enable us to make more
effective use of corpora are essential. In Chapter 3, we presented a way to add linguistic
annotation provided by a parser to a corpus in an xml format. The parsed version of the
bnc thus obtained was put into use in different ways in subsequent chapters, and has also
been used by other researchers for other purposes (recent papers mention work on semantic
role labelling, Fürstenau & Lapata 2009, and logical metonymy, Shutova 2009).
In Chapter 4, we presented a supervised learning approach to the binary sentence classification task and obtained an accuracy of over 70 per cent using pairs of correct and
incorrect sentences from the clc as both training and test data. Features including n-grams
of words and parts of speech as well as grammatical relations provided by the rasp system
contributed to this result, and it is expected that additional features, for instance labelled
grammatical relations, would permit this generic technique to perform even better. In order to distinguish more effectively between correct sentences and ones containing a single
minor mistake, more specialised error detectors can be useful. Enriching the set of features
with specific indicators of particular types of error was shown to give a significant increase
in accuracy, and this approach is eminently extensible to further error types.
It is difficult to evaluate our system’s performance against others’ since there is no generally
available test set. As we saw in Chapter 5, though, others have often relied on synthetic
data for training, which, according to our experiments, gives inferior performance on real
errors, although faux errors are of course better than none. As for the value of synthetic
data as a complement to real data for training purposes, further investigations are needed.
Synthetic data has occasionally been employed as test data; this approach, however, is
tantamount to the introduction of an artificial task, the performance on which may not be
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chapter 8. conclusion
elucidative of the system’s ability to handle ‘English as she is spoke’: a representative set of
real errors for testing being made publicly available would be an important step towards
enabling systems to be compared against each other (although part of the problem is of
course how to define such a set).
For errors that consist in an inappropriately chosen adjective or preposition, the clc is
often not on its own sufficient as a source of evidence; the preferred adjective or preposition in a given context is lexically idiosyncratic, which leads to data sparsity issues. In
Chapter 6, we presented encouraging results on the task of detecting adjective choice errors
based on statistics from the bnc, as well as state-of-the-art results on the task of detecting
certain types of preposition errors when accumulating statistics over contexts involving similar verbs/nouns. Only magnitude adjectives were included in the experiments, so further
work is needed to determine whether this approach can be extended to other classes of confusable adjectives, as well as whether clustering nouns according to adjectival preferences
will be useful for less common adjective–noun combinations. The method for detecting
prepositional choice errors can be extended to other frames (e.g., adjective–preposition–
noun and verb–preposition–noun), and it would be interesting to verify that this method,
aimed at finding errors that simpler lexically based systems would overlook, can usefully
complement a method that finds more trivial errors.
The idea of dedicated techniques for detection of specific errors, which may be more effective than generic approaches and furthermore allow the type of error to be identified and
perhaps a plausible correction to be provided, was implemented in Chapter 7 as part of a
system for semi-automatic error annotation. We showed that semi-automatic annotation
can be significantly more efficient than manual annotation whilst reducing the tedium of
marking up trivial errors over and over again and resulting in more consistent annotation.
Handling additional error types and eliminating frequent false positives would give a system that could potentially allow an error-annotated corpus to grow faster and reach higher
annotation consistency without additional resources.
Appendix a.
Error taxonomies
a.1. Gooficon classification
1. Clause skeleton
a) Missing parts
b) Misordered parts
2. Auxiliary system
a) Do
b) Have and be
c) Modals
d) Mismatch in tag questions
3. Passive
a) Formation
b) Use
4. Temporal conjunctions
a) Misplacement
b) Following clause type
c) Predicate type
d) Superficial tense agreement
5. Sentential complements
a) Misordering in subordinates
b) Subject extraposition
c) Infinitives and gerunds
d) Verb complement types
6. Psychological predicates
a) Subject/object misordering
b) Complements of reverse verbs
124
appendix a. error taxonomies
c) Straightforward adjectives
d) Reverse adjectives
(Source: Burt & Kiparsky 1972.)
a.2. FreeText error taxonomy
F
Form
AGL
MAJ
DIA
HOM
GRA
Agglutination
Upper/lower case
Diacritics
Homonymy
Other spelling errors
M
Morphology
MDP
MDS
MFL
MFC
MCO
Derivation-prefixation
Derivation-suffixation
Inflection
Inflection-confusion
Compounding
G
Grammar
CLA
AUX
GEN
MOD
NBR
PER
TPS
VOI
EUF
Class
Auxiliary
Gender
Mode
Number
Person
Tense
Voice
Euphony
L
Lexis
SIG
CPA
CPD
CPV
CPN
FIG
Meaning
Adjective complementation
Adverb complementation
Verb complementation
Noun complementation
Prefab
X
Syntax
ORD
MAN
RED
COH
Word order
Word missing
Word redundant
Cohesion
R
Register
RLE
RSY
Lexis
Syntax
Y
Style
CLR
LOU
Unclear
Heavy
Q
Punctuation
CON
TRO
OUB
Punctuation confusion
Punctuation redundant
Punctuation missing
Z
Typo
The error categories above are combined with a part-of-speech tag to form a complete error description.
(Source: Granger 2003.)
a.3. sst corpus taxonomy
125
a.3. SST corpus taxonomy
N
Noun
INF
NUM
CS
CNT
CMP
LXS
Inflection
Number
Case
Countability
Complement (of noun)
Lexis
V
Verb
INF
AGR
FML
TNS
ASP
VO
FIN
NG
QST
CMP
LXS
Inflection
Subject–verb disagreement
Form
Tense
Aspect
Voice
Usage of finite/infinite verb
Negation
Question
Complement (of verb)
Lexis
MO
Modal Verb
LXS
Lexis
AJ
Adjective
INF
US
NUM
AGR
QNT
CMP
LXS
Inflection
Usage of positive/comparative/superlative
Number
Number disagreement
Quantitative adjective
Complement (of adjective)
Lexis
AV
Adverb
INF
US
LXS
Inflection
Usage of positive/comparative/superlative
Lexis
PRP
Preposition
CMP
LXC1
LXC2
Complement (of preposition)
Normal preposition
Dependent preposition
AT
Article
AT
Article
PN
Pronoun
INF
AGR
CS
LXS
Inflection
Number/gender disagreement
Case
Lexis
CON
Conjunction
LXS
Lexis
REL
Relative pronoun
CS
LXS
Case
Lexis
ITR
Interrogative
LXS
Lexis
O
Others
JE
LXS
ODR
UK
UIT
Japanese English
Collocation
Misordering of words
Unknown type errors
Unintelligible utterance
(Source: Izumi, Uchimoto & Isahara 2004.)
126
appendix a. error taxonomies
a.4. ICLE/Louvain taxonomy of errors
F
Form
M
S
PW
PM
Morphology
Spelling
Punctuation wrong
Punctuation missing
G
Grammar
A
ADJCS
ADJN
ADJO
ADVO
NC
NN
P
VAUX
VM
VN
VNF
VT
VV
WC
Articles
Adjectives, comparative/superlative
Adjectives, number
Adjectives, order
Adverbs, order
Nouns, case
Nouns, number
Pronouns
Verbs, auxiliaries
Verbs, morphology
Verbs, number
Verbs, non-finite/finite
Verbs, tense
Verbs, voice
Word class
L
Lexis
CC
CLC
CLS
LC
P
S
SF
Conjunctions, coordinating
Connectors, logical, complex
Connectors, logical, single
Conjunctions, subordinating
Phrase
Single
Single, false friends
R
Register
[No subcategories]
S
Style
I
U
[Other]
Incomplete
Unclear
W
Word
M
O
R
Missing
Order
Redundant
X
Lexico-Grammar
ADJCO
ADJPR
CONJCO
NCO
NPR
NUC
PRCO
VCO
VPR
Adjectives, complementation
Adjectives, dependent preposition
Conjunction, complementation
Noun, complementation
Noun, dependent preposition
Noun, countable/uncountable
Preposition, complementation
Verb, complementation
Verb, dependent preposition
(Source: MacDonald Lightbound 2005.)
Appendix b.
Encoding systems
This appendix gives an overview of how characters as well as higher-level concepts such as italics, titles and
sentence boundaries have been encoded in different corpora: Brown gives the encoding used in the Brown
Corpus (Francis & Kučera 1964); LOB the one used in the Lancaster–Oslo/Bergen Corpus (Johansson,
Leech & Goodluck 1978); SGML the sgml entities used in the first edition of the British National Corpus
(Burnard 1995); and XML/Unicode the actual character or xml representation used in the last edition of the
British National Corpus (Burnard 2007).
The columns Brown, LOB and SGML only show the characters that actually occur in the respective corpora.
For accented letters, l represents any letter, and the XML/Unicode column illustrates the letter–accent combinations that occur in the bnc. The XML/Unicode column occasionally differ from the bnc xml edition by not
replicating manifest errors. The system used for transcribing Cyrillic letters in the lob corpus is not shown
below. There is much more mark-up above the character level than what is shown below.
Brown
LOB
SGML
XML/Unicode
1, 2, 3. . . .
*/1209*
*/1209*
1, 2, 3, . . .
*=1209
**=1209
1/2
1, 2, 3, . . .
MCCIX
mccix
&frac12
&half
&frac13
&frac14
&frac15
&frac16
&frac17
&frac18
&frac19
&frac23
&frac25
&frac34
&frac35
&frac38
&frac45
&frac47
&frac56
&frac58
1, 2, 3, . . .
MCCIX
mccix
1⁄2
1⁄2
1⁄3
1⁄4
1⁄5
1⁄6
1⁄7
1⁄8
1⁄9
2⁄3
2⁄5
3⁄4
3⁄5
3⁄8
4⁄5
4⁄7
5⁄6
5⁄8
128
appendix b. encoding systems
Brown
*A, *B, *C, . . .
A, B, C, . . .
LOB
A, B, C, . . .
a, b, c, . . .
ae
oe
ss
*?24
*?26
l*?2
l*?10
l*?6
l*?5
l*?3
l*?1
l*?15
*?19
L*?11
l*?11
O*?11
o*?11
l*?4
l**D
l"
*ZA
*YA
*ZB
*YB
*ZG
*YG
*ZD
*YD
*ZE
\15A
\15a
\15B
\15b
\15G
\15g
\15D
\15d
\15E
SGML
XML/Unicode
&frac78
&sup1
&sup2
&sup3
78
A, B, C, . . .
a, b, c, . . .
&AElig
&aelig
&eth
&OElig
&oelig
&szlig
&THORN
&thorn
A, B, C, . . .
a, b, c, . . .
Æ
æ
ð
Œ
œ
ß
Þ
þ
ȝ
&lacute
&acute
&abreve
&lcaron
&lcedil
ÁáćÉéÍı́ĹĺńÓóশÚúýź
´
ă
Ččd᾿ĚěňŘřŠšt᾿Žž
ÇçnŞşţ
&lcirc
&circ
&zdot
&lgrave
&grave
&lmacr
&logon
&lring
&dstrok
&hstrok
&Lstrok
&lstrok
&Oslash
&oslash
&ltilde
&tilde
&luml
&uml
&die
ÂâĉÊêÎı̂ÔôŝÛûŵŶŷ
ˆ
ż
ÀàÈèı̀òù
`
ĀāĒēı̄ōū
a˛e˛
Ååů
d̄
h̄
Ł
ł
Ø
ø
ãÕõÑñ
˜
ÄäËëÏı̈ÖöÜüŸÿ
¨
¨
⁄
1
2
3
᾿
A
&agr
&Bgr
&bgr
&Ggr
&ggr
&Dgr
&dgr
&Egr
α
B
β
Γ
γ
∆
δ
E
129
Brown
LOB
SGML
XML/Unicode
*YE
*ZZ
*YZ
*ZH
*YH
*ZJ
*YJ
*ZI
*YI
*ZK
*YK
*ZL
*YL
*ZM
*YM
*ZN
*YN
*ZX
*YX
*ZO
*YO
*ZP
*YP
*ZR
*YR
*ZS
*YS
*ZT
*YT
*ZU
*YU
*ZF
*YF
*ZC
*YC
*ZY
*YY
*ZQ
*YQ
\15e
\15Z
\15z
\15E
\15e
{15TH}
{15th}
\15I
\15i
\15K
\15k
\15L
\15l
\15M
\15m
\15N
\15n
\15X
\15x
\15O
\15o
\15P
\15p
\15R
\15r
\15S
\15s
\15T
\15t
\15U
\15u
\15F
\15f
{15CH}
{15cH}
{15PS}
{15ps}
\15O
\15o
&egr
&Zgr
&zgr
ε
&khgr
&PSgr
&psgr
&OHgr
&ohgr
&ohm
χ
**A
**C
**S
**I
**X
.
,
-
’
:
;
?
!
.
,
-
’
:
;
?
!
.
,
-
’
:
;
?
!
.
,
-
Z
ζ
H
&eegr
&THgr
&thgr
η
Θ
θ
I
&igr
ι
K
&kgr
κ
Λ
&lgr
&Mgr
&mgr
λ
M
µ
N
&ngr
ν
Ξ
&xgr
&Ogr
&ogr
&Pgr
&pgr
ξ
O
ο
Π
π
P
&rgr
&Sgr
&sgr
ρ
Σ
σ
T
&tgr
&Ugr
&ugr
&PHgr
&phgr
τ
Y
υ
Φ
φ
X
Ψ
ψ
Ω
ω
Ω
130
appendix b. encoding systems
Brown
LOB
SGML
XML/Unicode
(
)
*
=
**K
+
+
(
)
*/
(
)
&ast
&equals
&percnt
&plus
&amp
&sol
&horbar
&lowbar
&dash
&ndash
&mdash
&ape
&bsol
&bull
&cent
&check
(
)
*
=
%
+
&amp; for &
/
—
&cir
&commat
&copy
&darr
&deg
&divide
&dollar
&dtrif
&flat
&ft
&ge
&gt
&Gt
&hearts
&hellip
&iexcl
&infin
&ins
&iquest
&larr
&lcub
&le
&lsqb
&lt
&Lt
&micro
&middot
&natur
&num
&plusmn
◦
@
©
↓
°
÷
$
t
**-
*+0
**?18
*?16
*@
*?25
*(
.
*?14
—
–
—
≈
\
•
¢
p
b
≥
&gt; or >
>>
¤
...
¡
∞
¿
←
{
≤
[
&lt; for <
<<
µ
·
n
#
±
131
Brown
LOB
SGML
XML/Unicode
*+
*?7
*?8
*?9
&pound
&prime
&Prime
£
*?22
*?23
*)
*?13
*?17
x
**Q
**U
**F
**N major heading **P
**R minor heading **T
*= italics *$
**= bold face **$
**( capitalised **)
*" or *’
**" or **’
**[FORMULA**]
ˆ sentence
∼ included sent.
list
| paragraph
*< heading *>
*0 roman
*1 italics
*4 bold face
*2 capitalised
**[ comment **]
&quot
&radic
&rarr
&rcub
&reg
&rsqb
&sect
&sharp
&sim
&shilling
&times
&trade
&verbar
&yen
&quot; or "
p
&bquo
&equo
&formula
&rehy
“
”
<gap desc="formula"/>
-
→
}
®
]
§
#
∼
/×
™
|
¥
<s>sentence</s>
<list>list</list>
<p>paragraph</p>
<head>heading</head>
<hi r=ro>roman</hi>
<hi r="it">italics</hi>
<hi r="bo">bold</hi>
<!-- comment -->
Appendix c.
Part-of-speech tags
The following table gives a brief overview of different sets of part-of-speech tags: Brown is the tagset used in
the tagged Brown Corpus (Francis & Kučera 1964); LOB, the tagset used in the tagged London–Oslo/Bergen
Corpus (Johansson & al. 1986); C530 , the tagset used in the British National Corpus; C231 , the tagset used
by rasp; C732 , an updated version of C2; and W, a simplified set of word classes (Burnard 2007) added to
the last edition of the bnc. Only the correspondences between W, C5 and C7 are authoritative; the mapping
between C2 and C7 should be fairly good given the similarity between the tagsets; the correspondence with
Brown and LOB is only approximate, however, and there are many differences which do not appear in the
synoptic table below.
W
C5
NN0
NN0
NN0
SUBST
NN1
NN1
NN1
NN1
NN2
NN2
NN2
NN2
NN2
NN2
NP0
NP0
NP0
NP0
NP0
NP0
Brown
LOB
(
NN
NN Œ
NNU
NR
(
NN
NN Œ
NNU
NNS









NNS Œ
(NNUS
NRS
NN-TL
NNS-TL
(
NPT
NPTS
(
NN-TL
NPT
NNS-TL
NP
NPTS
NPL
30
) http://ucrel.lancs.ac.uk/claws5tags.html
31
) http://ucrel.lancs.ac.uk/claws2tags.html
32
) http://ucrel.lancs.ac.uk/claws7tags.html
C2
C7
NN
NNJ
NNU
common noun sg./pl.: sheep, cod
organisation: department, council
unit of measurement: in., cc.
ND1
NN1
NNT1
NNU1
direction: north, southeast
singular: book, girl
temporal noun sg.: day, week, year
unit, sg.: inch, centimetre
NN2
NNJ2
NNT2
NNU2
NPD2
NPM2
(
NNSA1
NNA
NNSA2
NNSB
NNB Œ
NNSB1
NNSB2
NNL1
pl. common noun: books, girls
pl. org.: governments, committees
temporal pl.: days, weeks, years
unit, pl.: inches, centimetres
weekday, pl.: Sundays
month, pl.: Octobers
following title, sg.: MA
following title, pl.
preceding title: Rt. Hon.
preceding title, sg.: Prof.
preceding title, pl.: Messrs
locative, sg.: Street, Bay
134
W
appendix c. part-of-speech tags
C5
Brown
NP0
NP0
NP0
NP0
NP0
NP0
NPS
LOB
VERB
NNL2
NP
NP1
NP2
NPD1
NPM1
NP
NPS(
NR
(
(
VBB Œ
BEM
BER
BEZ
BED
BEDZ
(
VBZ(
VBD
VHI
VHB
VHZ
VHD
MD
BEG
HVG
VBG Œ
BEN
HVN
VBN Œ
VVN
JJS





JJ




(
VVZ
VVD
infinitive: take, live
finite base form: take, live
-s form: takes, lives
past tense: took, lived
VM
VMK
modal: can, could
catenative: ought, used (to)
VBG
VHG
VDG
VVG
VVGK
being
having
doing
-ing part.: giving, working
catenative: (be) going (to)
VBN
VHN
VDN
VVN
VVNK
been
had past part.
done
past part.: given, worked
catenative: (be) bound (to)
VVI
VV0
VBZ
VBD
(
VBN
VHN
VDN(
do inf.
do fin.
does
did
VD0
VDZ
VDD
VB
VVG
(
VDI
VD0
DOZ
DOD
(
BEG
VHG
VDG(
have inf.
have fin.
has
had past tense
VH0
VHZ
VHD
DO
VM0
(
VHI
VH0
HVZ
HVD
(
VVI
VVB
VVZ
VVD
(
be inf.
be subj./imp.
am
are, art
is
were
was
VB0
VBM
VBR
VBZ
VBDR
VBDZ
HV
VDI
VDB
VDZ
VDD
Letter sg.: A, a, B, π
Letter pl.: As, a’s, Bs
(
VBI
VB0
BE
locative, pl.: islands, roads
proper noun: Indies, Andes
proper noun sg.: London, Jane
proper noun pl.: Browns, Reagans
weekday, sg.: Sunday
month, sg.: October
ZZ1
ZZ2
ZZ
VBI
ADJ
C7
( NPLS
ZZ0
ZZ0
AJ0
AJ0
AJ0
AJ0
AJ0
C2
JJ Œ
JJB
JJ
JJ
VV0
(





JJ
JA
JB




JK
semantic superlative: chief, top, utmost
general adjective
predicative: tantamount, asleep
attributive: main, chief, utter
catenative: (be) able, willing (to)
135
W
C5
Brown
AJC
AJC
AJC
AJC
CRD
CRD
CRD
CRD
CRD
CRD
CRD
CRD
JJR
CD
(
CD1
CD Œ
(
CDS












CD$
CD-CD
MC1
MC
NNO
MF
NNO2
MC2
MCGE
MC$
MCMC MC-MC
OD

















CS
TO
after-det.: such, former, same
singular: little, much
plural: few, several, many
comparative: more, less
plural: fewer
superlative: most, least
before-det.: all, half
plural: both
determiner: any, some
singular: this, that, another
plural: these, those
DAR
DA2R
DAT
DB
DB2
DD
DD1
DD2
(
BCL
BCS
BTO
RG
RGA
RGR
RGT
RL
RR
RRR
RRT
RT
RP
RPK
RGQ
RGQV
RRQ
RRQV
RN
RB
RBR
RBT
RN(
RP
WQL
WDT
WRB
WDT
*
DA
DA1
DA2
RG
QL
XNOT
one
cardinal number: two, three
numeral noun: dozen, thousand
fraction: quarters, two-thirds
hundreds, thousands
tens, twenties
genitive: 10’s
hyph’d number: 40-50, 1770-1827
ordinal numeral: first, second
RA
REX
(
QL
QLP
(
comparative: older, better, bigger
attr. comp.: upper, outer
superlative: oldest, best, biggest
attr. sup.: utmost, uttermost
MD
DAR
ABN
ABX
DTI
DT
DTS
AV0
AV0
AV0
AV0
AV0
AV0
AV0
AV0
AV0
AV0
AV0
AV0
AV0
AVP
AVP
AVQ
AVQ
AVQ
AVQ
JJR
JBR
JJT
JBT
JJT













AP
C7
JJR
JJT
DT0
DT0
DT0
DT0
DT0
DT0
DT0
DT0
DT0
DT0
DT0
ADV
C2
(
(
ORD
XX0
LOB
(
XX
before conj.: in order (that), even (if)
before inf. marker: in order, so as (to)
post nominal: else, galore
appos. introd.: namely, viz, e.g.
degree adv: very, so, to
post-nominal/adv./adj.: indeed, enough
comparative degree adv.: more, less
superlative degree adv.: most, least
locative: alongside, forward
general adverb
comparative: better, longer
superlative: best, longest
nominal adv. of time: now, tomorrow
prep. adv./particle: in, up, about
catenative: (be) about (to)
wh- degree adv.: how
wh-ever degree adv.: however
wh- general adv.: where, when
wh-ever general adv.: wherev., whenev.
not, n’t
appendix c. part-of-speech tags
W
C5
ART
136
AT0
Brown
LOB
(
AT
AT1
(
AT
PRON
DPS
DTQ
DTQ
DTQ
WDT(R)
WP$
PREP
PNX
PNX
PNX
PRF
PRP
PRP
PRP
TO0
WP$(R)
EX
(
PNI
PNI
PNQ
PNQ
PNQ
PNQ
PNQ
APPGE
(
WDT
PN
(
PP3
PP3A
PP1A
PP1AS
PP2
PP3AS
PP3O
PP1O
PP1OS
PP3OS
PPS
PPSS
PPO


















PP$$
WPS
WPO
WPS
WPO
WP$
WP(A)(R)
WPO(R)
WPA(R)
WPO(R)
WP$(R)
(
PPL
PPLS
IN









TO
C7
AT
AT1
PP$
(
EX0
PNP
PNP
PNP
PNP
PNP
PNP
PNP
PNP
PNP
PNP
PNP
C2
the, no
a, an
APP$
DDQ
DDQV
DDQGE
DDQ$
genitive pron.: my, your, our
wh-determiner: which, what
wh-ever determiner: whichever
whose
EX
there existential
PN
PN1
indef. pron.: none
sing.: anyone, everything
PPH1
PPHS1
PPIS1
PPIS2
PPY
PPHS2
PPHO1
PPIO1
PPIO2
PPHO2
PPGE
PP$
it
he, she
I
we
you
they
him, her
me
us
them
possessive pron.: mine, ours
PNQS
PNQO
PNQVS
PNQV
PNQVO
PNQV$
who
whom
who(so)ever
whom(so)ever
whosever
PNX1
PPX1
PPX2
oneself
reflexive sg.: yourself, itself
reflexive pl.: yourselves, ourselves
II
IF
IO
IW
preposition
for
of
with
TO
to inf. marker
137
C5
INTERJ
UNC
UNC
UNC
POS
PUN
PUN
PUN
PUN
PUN
PUN
PUL
PUR
PUN
PUN
PUQ
LOB
C2
(
CS
(









CSA
CSN
CSW
CST
UH
UH
&FO
&FW
$
.





FO
FU
FW
&FW
,
:
(
)
—
...
*’ **’
$
.
;
!
?
,
:
(
)
—
...
"
semi-coordinating: so, then, yet
subordinating: if, because, unless
as
than
whether
that
interjection: oh, yes, um
&FO
GE
.
;
!
?




coordinating: and, or
but
CF
CS
CS








FW-
C7
CC
CCB
CC
CJS
CJS
CJS
CJS
CJS
CJT
ITJ
STOP
Brown
CJC
CJC
UNC
CONJ
W
Formula
Unknown
Foreign word
Genitive marker
Full stop
Semicolon
Exclamation mark
Question mark
Comma
Colon
Opening bracket
Closing bracket
Dash
Ellipsis
Quotation mark
Appendix d.
XML listings
d.1. CLC mark-up from Chapter 3
<sent idx="247">
<orig idx="247">Then <NS type="RD">some|a </NS> <NS type="SX">though|thought </NS>
<NS type="IV">occured|occurred </NS> to me. </orig>
<correct idx="247" partnum="1">
<part num="1">
<c orig>Then <NS type="RD">some|a </NS> <NS type="SX">though|thought </NS>
<NS type="IV">occured|occurred </NS> to me. </c orig>
<c text>Then a thought occurred to me. </c text>
<c rasp>
<string><![CDATA[(|Then:1 RR| |a:2 AT1| |thought:3 NN1| |occur+ed:4 VVD|
|to:5 II| |I+:6 PPIO1| |.:7 .|) 1 ; (-9.204) ]]></string>
<parse>
<tree>
<node rule="T/txt-sc1/-+">
<node rule="S/adv s/-">
<node rule="AP/a1">
<node rule="A1/a">
<wnode>
<pos>RR</pos>
<word>Then</word>
</wnode>
</node>
</node>
<node rule="S/np vp">
<node rule="NP/det n1">
<wnode>
<pos>AT1</pos>
<word>a</word>
</wnode>
<node rule="N1/n">
<wnode>
<pos>NN1</pos>
140
appendix d. xml listings
<word>thought</word>
</wnode>
</node>
</node>
<node rule="V1/v pp">
<wnode>
<pos>VVD</pos>
<word>occur+ed</word>
</wnode>
<node rule="PP/p1">
<node rule="P1/p np-pro">
<wnode>
<pos>II</pos>
<word>to</word>
</wnode>
<wnode>
<pos>PPIO1</pos>
<word>I+</word>
</wnode>
</node>
</node>
</node>
</node>
</node>
<node rule="End-punct3/-">
<wnode>
<pos>.</pos>
<word>.</word>
</wnode>
</node>
</node>
</tree>
<gr-list>
<gr>
<gr-type>ncmod</gr-type>
<subtype> </subtype>
<head><pos>VVD</pos><word>occur+ed</word></head>
<dep><pos>RR</pos><word>Then</word></dep>
</gr>
<gr>
<gr-type>ncsubj</gr-type>
<head><pos>VVD</pos><word>occur+ed</word></head>
<dep><pos>NN1</pos><word>thought</word></dep>
<init-gr> </init-gr>
</gr>
<gr>
<gr-type>iobj</gr-type>
<head><pos>VVD</pos><word>occur+ed</word></head>
<dep><pos>II</pos><word>to</word></dep>
d.1. clc mark-up from chapter 3
141
</gr>
<gr>
<gr-type>dobj</gr-type>
<head><pos>II</pos><word>to</word></head>
<dep><pos>PPIO1</pos><word>I+</word></dep>
</gr>
<gr>
<gr-type>det</gr-type>
<head><pos>NN1</pos><word>thought</word></head>
<dep><pos>AT1</pos><word>a</word></dep>
</gr>
</gr-list>
</parse>
</c rasp>
</part>
</correct>
<incorrect idx="247" partnum="1">
<part num="1">
<inc orig>Then <NS type="RD">some|a </NS> <NS type="SX">though|thought </NS>
<NS type="IV">occured|occurred </NS> to me. </inc orig>
<inc text>Then some though occured to me. </inc text>
<inc rasp>
<string><![CDATA[(|Then:1 RR| |some:2 DD| |though:3 RR| |occur+ed:4 VVN|
|to:5 II| |I+:6 PPIO1| |.:7 .|) 1 ; (-9.738) ]]></string>
<parse>
<tree>
<node rule="T/txt-sc1/-+">
<node rule="S/adv s/-">
<node rule="AP/a1">
<node rule="A1/a">
<wnode>
<pos>RR</pos>
<word>Then</word>
</wnode>
</node>
</node>
<node rule="S/np vp">
<wnode>
<pos>DD</pos>
<word>some</word>
</wnode>
<node rule="V1/adv vp">
<node rule="AP/a1">
<node rule="A1/a">
<wnode>
<pos>RR</pos>
<word>though</word>
</wnode>
</node>
142
appendix d. xml listings
</node>
<node rule="V1/v pp">
<wnode>
<pos>VVN</pos>
<word>occur+ed</word>
</wnode>
<node rule="PP/p1">
<node rule="P1/p np-pro">
<wnode>
<pos>II</pos>
<word>to</word>
</wnode>
<wnode>
<pos>PPIO1</pos>
<word>I+</word>
</wnode>
</node>
</node>
</node>
</node>
</node>
</node>
<node rule="End-punct3/-">
<wnode>
<pos>.</pos>
<word>.</word>
</wnode>
</node>
</node>
</tree>
<gr-list>
<gr>
<gr-type>ncmod</gr-type>
<subtype> </subtype>
<head><pos>VVN</pos><word>occur+ed</word></head>
<dep><pos>RR</pos><word>Then</word></dep>
</gr>
<gr>
<gr-type>ncsubj</gr-type>
<head><pos>VVN</pos><word>occur+ed</word></head>
<dep><pos>DD</pos><word>some</word></dep>
<init-gr> </init-gr>
</gr>
<gr>
<gr-type>ncmod</gr-type>
<subtype> </subtype>
<head><pos>VVN</pos><word>occur+ed</word></head>
<dep><pos>RR</pos><word>though</word></dep>
</gr>
d.2. bnc mark-up from chapter 3
143
<gr>
<gr-type>iobj</gr-type>
<head><pos>VVN</pos><word>occur+ed</word></head>
<dep><pos>II</pos><word>to</word></dep>
</gr>
<gr>
<gr-type>dobj</gr-type>
<head><pos>II</pos><word>to</word></head>
<dep><pos>PPIO1</pos><word>I+</word></dep>
</gr>
</gr-list>
</parse>
</inc rasp>
</part>
</incorrect>
</sent>
d.2. BNC mark-up from Chapter 3
d.2.1. Original mark-up
The example sentence as it appears in the bnc. (This particular sentence is not actually part of the corpus, but
has been assembled from parts which are.)
<s n="1">
<trunc><w c5="UNC" hw="any" pos="UNC">Any </w></trunc>
<w c5="PNI" hw="anyone" pos="PRON">anyone </w>
<w c5="PNQ" hw="who" pos="PRON">who </w>
<w c5="AJ0-VVN" hw="dissolved" pos="ADJ">dissolved </w>
<mw c5="AV0">
<w c5="AV0" hw="more" pos="ADV">more </w>
<w c5="CJS" hw="than" pos="CONJ">than </w>
</mw>
<w c5="UNC" hw="½" pos="UNC">½ </w>
<gap desc="formula"/>
<w c5="PRP" hw="in" pos="PREP"> in </w>
<w c5="NN1" hw="rivers/lakes" pos="SUBST">rivers/lakes </w>
<w c5="VBZ" hw="be" pos="VERB">is</w>
<w c5="XX0" hw="not" pos="ADV">n’t </w>
<w c5="VVG" hw="gon" pos="VERB">gon</w>
<w c5="TO0" hw="na" pos="PREP">na </w>
<w c5="VVI" hw="forget" pos="VERB">forget </w>
<w c5="DPS" hw="his/her" pos="PRON">his </w>
<w c5="NN1" hw="pilgrimage" pos="SUBST">pilgrimage</w>
<c c5="PUN">, </c>
<w c5="VVB-NN1" hw="y’know" pos="VERB">y’know</w>
<c c5="PUN">.</c>
</s>
144
appendix d. xml listings
d.2.2. After parsing
The example sentence after parsing with the current version of rasp4uima. Added elements and attributes are
printed in italics. Note that the attributes n, rpos, lem and affix are space-separated lists when tokens have
been split; in particular, the affix attribute for rivers/lakes has the value "+s +s" with two spaces.
<s n="1">
<trunc><w c5="UNC" hw="any" pos="UNC">Any </w></trunc>
<w n="1" c5="PNI" hw="anyone" pos="PRON" rpos="PN1" lem="anyone">anyone </w>
<w n="2" c5="PNQ" hw="who" pos="PRON" rpos="PNQS" lem="who">who </w>
<w n="3" c5="AJ0-VVN" hw="dissolved" pos="ADJ"
rpos="VVD" lem="dissolve" affix="+ed">dissolved </w>
<mw c5="AV0">
<w n="4" c5="AV0" hw="more" pos="ADV" rpos="DAR" lem="more">more </w>
<w n="5" c5="CJS" hw="than" pos="CONJ" rpos="CSN" lem="than">than </w>
</mw>
<w n="6" c5="UNC" hw="½" pos="UNC" rpos="MC" lem="½">½ </w>
<gap n="7" desc="formula" rpos="&amp;FO" lem="[gap]"/>
<w n="8" c5="PRP" hw="in" pos="PREP" rpos="II" lem="in"> in </w>
<w n="9 10 11" c5="NN1" hw="rivers/lakes" pos="SUBST"
rpos="NNL2 CC NN2" lem="river / lake" affix="+s +s">rivers/lakes </w>
<w n="12" c5="VBZ" hw="be" pos="VERB" rpos="VBZ" lem="be" affix="+s">is</w>
<w n="13" c5="XX0" hw="not" pos="ADV" rpos="XX" lem="not" affix="+">n’t </w>
<w n="14" c5="VVG" hw="gon" pos="VERB" rpos="VVN" lem="gon">gon</w>
<w n="15" c5="TO0" hw="na" pos="PREP" rpos="TO" lem="na">na </w>
<w n="16" c5="VVI" hw="forget" pos="VERB" rpos="VV0" lem="forget">forget </w>
<w n="17" c5="DPS" hw="his/her" pos="PRON" rpos="APP$" lem="his">his </w>
<w n="18" c5="NN1" hw="pilgrimage" pos="SUBST" rpos="NN1" lem="pilgrimage">pilgrimage</w>
<c n="19" c5="PUN" rpos="," lem=",">, </c>
<w n="20 21" c5="VVB-NN1" hw="y’know" pos="VERB"
rpos="PPY VV0" lem="y’ know">y’know</w>
<c n="22" c5="PUN" rpos="." lem=".">.</c>
<grlist parse="1" score="-40.848">
<gr type="ncsubj" head="14" dep="1"/>
<gr type="cmod" subtype=" " head="1" dep="3"/>
<gr type="ncsubj" head="3" dep="2"/>
<gr type="ncmod" subtype=" " head="3" dep="8"/>
<gr type="xcomp" subtype=" " head="3" dep="4"/>
<gr type="ncmod" subtype=" " head="4" dep="5"/>
<gr type="dobj" head="5" dep="7"/>
<gr type="ncmod" subtype=" " head="7" dep="6"/>
<gr type="dobj" head="8" dep="10"/>
<gr type="conj" head="10" dep="9"/>
<gr type="conj" head="10" dep="11"/>
<gr type="aux" head="14" dep="12"/>
<gr type="ncmod" subtype=" " head="14" dep="13"/>
<gr type="ta" subtype="end" head="14" dep="21"/>
<gr type="xcomp" subtype="to" head="14" dep="16"/>
<gr type="passive" head="14"/>
<gr type="dobj" head="16" dep="18"/>
d.3. from chapter 7
145
<gr type="det" head="18" dep="17"/>
<gr type="ncsubj" head="21" dep="20"/>
</grlist>
</s>
d.3. From Chapter 7
d.3.1. example.xml
<!DOCTYPE learner SYSTEM "LNRC.dtd">
<learner>
<head url="1865831">
<candidate>
<exam>
<exam code>0085</exam code>
<exam desc>Key English Test</exam desc>
<exam level>KET</exam level> </exam>
<personnel>
<language>Spanish</language>
. . . </personnel>
<scores and grades>
. . . </scores and grades> </candidate>
<text>
<answer1>
<question number>9</question number>
<exam score>4</exam score>
<original answer>
<p>We can not stomach an other christmastime
celebration with all kind of delicacys for sweet
tooths.</p>
<p>In the other hand, it would be also a sham too
disappoint an hopefull childly heart.</p>
</original answer> </answer1> </text> </head>
<head url="2845832">
<candidate>
<exam>
<exam code>0100</exam code>
<exam desc>First Certificate of English</exam desc>
<exam level>FCE</exam level> </exam>
<personnel>
<language>Italian</language>
. . . </personnel>
<scores and grades>
. . . </scores and grades> </candidate>
<text>
<answer1>
<question number>7</question number>
<exam score>5</exam score>
146
appendix d. xml listings
<original answer>
<p>The cr&egrave;me br&ucirc;l&eacute;e costed
&pound;18 at Mangiare &amp; Bere.</p>
</original answer> </answer1> </text> </head> </learner>
d.3.2. example.xml.0
<?xml version="1.0"?>
<rasp>
<p>We can not stomach an other christmastime celebration with all kind of delicacys for sweet
tooths.</p>
<p>In the other hand, it would be also a sham too disappoint an hopefull childly heart.</p>
<p>The crème brûlée costed £18 at Mangiare &amp; Bere.</p>
</rasp>
d.3.3. example.xml.0tag
<?xml version="1.0"?>
<rasp>
<p>We <e t="RP 0rp"><i>can not</i><c>cannot</c></e> stomach
<e t="RP 0rp"><i>an other</i><c>another</c></e> christmastime celebration with all
<e t="AGN 0agn"><i>kind</i><c>kinds</c></e> of delicacys for sweet tooths.</p>
<p><e t="ID 0id"><i>In the other</i><c>On the other</c></e> hand, it would
<e t="W 0w"><i>be also</i><c>also be</c></e> a sham too disappoint an
<e t="DJ 0dj"><i>hopefull</i><c>hopeful</c></e> childly heart.</p>
<p>The crème brûlée <e t="IV 0iv"><i>costed</i><c>cost</c></e> £18 at Mangiare &amp; Bere.</p>
</rasp>
d.3.4. example.xml.2
<?xml version="1.0"?>
<rasp>
<p>We cannot stomach another christmastime celebration with all kinds of delicacys for sweet
tooths.</p>
<p>On the other hand, it would also be a sham too disappoint an hopeful childly heart.</p>
<p>The crème brûlée cost £18 at Mangiare &amp; Bere.</p>
</rasp>
d.3.5. example.xml.2token
<rasp>
<p>
<start-s n="0"/> ˆ <w s=’0’ e=’1’>We</w> <w s=’3’ e=’8’>cannot</w>
d.3. from chapter 7
147
<w s=’10’ e=’16’>stomach</w> <w s=’18’ e=’24’>another</w> <w s=’26’ e=’38’>christmastime</w>
<w s=’40’ e=’50’>celebration</w> <w s=’52’ e=’55’>with</w> <w s=’57’ e=’59’>all</w>
<w s=’61’ e=’65’>kinds</w> <w s=’67’ e=’68’>of</w> <w s=’70’ e=’78’>delicacys</w>
<w s=’80’ e=’82’>for</w> <w s=’84’ e=’88’>sweet</w> <w s=’90’ e=’95’>tooths</w>
<w s=’96’ e=’96’>.</w> ˆˆ <end-s n="96"/>
</p>
<p>
<start-s n="0"/> ˆ <w s=’0’ e=’1’>On</w> <w s=’3’ e=’5’>the</w> <w s=’7’ e=’11’>other</w>
<w s=’13’ e=’16’>hand</w> <w s=’17’ e=’17’>,</w> <w s=’19’ e=’20’>it</w>
<w s=’22’ e=’26’>would</w> <w s=’28’ e=’31’>also</w> <w s=’33’ e=’34’>be</w>
<w s=’36’ e=’36’>a</w> <w s=’38’ e=’41’>sham</w> <w s=’43’ e=’45’>too</w>
<w s=’47’ e=’56’>disappoint</w> <w s=’58’ e=’59’>an</w> <w s=’61’ e=’67’>hopeful</w>
<w s=’69’ e=’75’>childly</w> <w s=’77’ e=’81’>heart</w> <w s=’82’ e=’82’>.</w> ˆˆ
<end-s n="82"/>
</p>
<p>
<start-s n="0"/> ˆ <w s=’0’ e=’2’>The</w> <w s=’4’ e=’8’>crème</w> <w s=’10’ e=’15’>brûlée</w>
<w s=’17’ e=’20’>cost</w> <w s=’22’ e=’22’>£</w> <w s=’23’ e=’24’>18</w>
<w s=’26’ e=’27’>at</w> <w s=’29’ e=’36’>Mangiare</w> <w s=’38’ e=’38’>&amp;</w>
<w s=’40’ e=’43’>Bere</w> <w s=’44’ e=’44’>.</w> ˆˆ<end-s n="44"/>
</p>
</rasp>
d.3.6. example.xml.2tag
<?xml version="1.0"?>
<rasp>
<p>We <e t="RP 0rp"><i>can not</i><c>cannot</c></e> stomach
<e t="RP 0rp"><i>an other</i><c>another</c></e>
<e t="RP 2rp">i>christmastime</i><c>Christmastime</c></e> celebration with all
<e t="AGN 0agn"><i>kind</i><c>kinds</c></e> of
<e t="IN 2in"><i>delicacys</i><c>delicacies</c></e> for sweet
<e t="IN 2in"><i>tooths</i><c>teeth</c></e>.</p>
<p><e t="ID 0id"><i>In the other</i><c>On the other</c></e> hand, it would
<e t="W 0w"><i>be also</i><c>also be</c></e> a sham too disappoint an
<e t="DJ 0dj"><i>hopefull</i><c>hopeful</c></e> <e t="S 2s"><i>childly</i><c>(¿?)</c></e>
heart.</p>
<p>The crème brûlée <e t="IV 0iv"><i>costed</i><c>cost</c></e> £18 at Mangiare &amp; Bere.</p>
</rasp>
d.3.7. example.xml.3
<?xml version="1.0"?>
<rasp>
<p>We cannot stomach another Christmastime celebration with all kinds of delicacies for sweet
teeth.</p>
148
appendix d. xml listings
<p>On the other hand, it would also be a sham too disappoint an hopeful (¿?) heart.</p>
<p>The crème brûlée cost £18 at Mangiare &amp; Bere.</p>
</rasp>
d.3.8. example.xml.3pos
<?xml version="1.0"?>
<rasp>
<p>
<start-s n="0"/>
<sentence num=’1’>
<lemma-list>
<lemma s=’0’ e=’1’ lem=’We’ num=’1’ wnum=’1’ pos=’PPIS2’/>
<lemma s=’3’ e=’8’ lem=’cannot’ num=’2’ wnum=’2’ pos=’VM’/>
<lemma s=’10’ e=’16’ lem=’stomach’ num=’3’ wnum=’3’ pos=’VV0’/>
<lemma s=’18’ e=’24’ lem=’another’ num=’4’ wnum=’4’ pos=’DD1’/>
<lemma s=’26’ e=’38’ lem=’Christmastime’ num=’5’ wnum=’5’ pos=’NN1’/>
<lemma s=’40’ e=’50’ lem=’celebration’ num=’6’ wnum=’6’ pos=’NN1’/>
<lemma s=’52’ e=’55’ lem=’with’ num=’7’ wnum=’7’ pos=’IW’/>
<lemma s=’57’ e=’59’ lem=’all’ num=’8’ wnum=’8’ pos=’DB2’/>
<lemma s=’61’ e=’65’ lem=’kinds’ num=’9’ wnum=’9’ pos=’NN2’/>
<lemma s=’67’ e=’68’ lem=’of’ num=’10’ wnum=’10’ pos=’IO’/>
<lemma s=’70’ e=’79’ lem=’delicacies’ num=’11’ wnum=’11’ pos=’NN2’/>
<lemma s=’81’ e=’83’ lem=’for’ num=’12’ wnum=’12’ pos=’IF’/>
<lemma s=’85’ e=’89’ lem=’sweet’ num=’13’ wnum=’13’ pos=’JJ’/>
<lemma s=’91’ e=’95’ lem=’teeth’ num=’14’ wnum=’14’ pos=’NN2’/>
<lemma s=’96’ e=’96’ lem=’.’ num=’15’ wnum=’15’ pos=’.’/>
</lemma-list>
</sentence>
<end-s n="96"/>
</p>
<p>
<start-s n="0"/>
<sentence num=’2’>
<lemma-list>
<lemma s=’0’ e=’1’ lem=’On’ num=’1’ wnum=’1’ pos=’II’/>
<lemma s=’3’ e=’5’ lem=’the’ num=’2’ wnum=’2’ pos=’AT’/>
<lemma s=’7’ e=’11’ lem=’other’ num=’3’ wnum=’3’ pos=’JB’/>
<lemma s=’13’ e=’16’ lem=’hand’ num=’4’ wnum=’4’ pos=’NN1’/>
<lemma s=’17’ e=’17’ lem=’,’ num=’5’ wnum=’5’ pos=’,’/>
<lemma s=’19’ e=’20’ lem=’it’ num=’6’ wnum=’6’ pos=’PPH1’/>
<lemma s=’22’ e=’26’ lem=’would’ num=’7’ wnum=’7’ pos=’VM’/>
<lemma s=’28’ e=’31’ lem=’also’ num=’8’ wnum=’8’ pos=’RR’/>
<lemma s=’33’ e=’34’ lem=’be’ num=’9’ wnum=’9’ pos=’VB0’/>
<lemma s=’36’ e=’36’ lem=’a’ num=’10’ wnum=’10’ pos=’AT1’/>
<lemma s=’38’ e=’41’ lem=’sham’ num=’11’ wnum=’11’ pos=’NN1’/>
<lemma s=’43’ e=’45’ lem=’too’ num=’12’ wnum=’12’ pos=’RG’/>
<lemma s=’47’ e=’56’ lem=’disappoint’ num=’13’ wnum=’13’ pos=’VV0’/>
d.3. from chapter 7
149
<lemma s=’58’ e=’59’ lem=’an’ num=’14’ wnum=’14’ pos=’AT1’/>
<lemma s=’61’ e=’67’ lem=’hopeful’ num=’15’ wnum=’15’ pos=’JJ’/>
<lemma s=’69’ e=’72’ lem=’(¿?)’ num=’16’ wnum=’16’ pos=’NN1’/>
<lemma s=’74’ e=’78’ lem=’heart’ num=’17’ wnum=’17’ pos=’NN1’/>
<lemma s=’79’ e=’79’ lem=’.’ num=’18’ wnum=’18’ pos=’.’/>
</lemma-list>
</sentence>
<end-s n="79"/>
</p>
<p>
<start-s n="0"/>
<sentence num=’3’>
<lemma-list>
<lemma s=’0’ e=’2’ lem=’The’ num=’1’ wnum=’1’ pos=’AT’/>
<lemma s=’4’ e=’8’ lem=’crème’ num=’2’ wnum=’2’ pos=’NN1’/>
<lemma s=’10’ e=’15’ lem=’brûlée’ num=’3’ wnum=’3’ pos=’NN1’/>
<lemma s=’17’ e=’20’ lem=’cost’ num=’4’ wnum=’4’ pos=’VVD’/>
<lemma s=’22’ e=’22’ lem=’£’ num=’5’ wnum=’5’ pos=’NNU’/>
<lemma s=’23’ e=’24’ lem=’18’ num=’6’ wnum=’6’ pos=’MC’/>
<lemma s=’26’ e=’27’ lem=’at’ num=’7’ wnum=’7’ pos=’II’/>
<lemma s=’29’ e=’36’ lem=’Mangiare’ num=’8’ wnum=’8’ pos=’NP1’/>
<lemma s=’38’ e=’38’ lem=’&amp;amp;’ num=’9’ wnum=’9’ pos=’CC’/>
<lemma s=’40’ e=’43’ lem=’Bere’ num=’10’ wnum=’10’ pos=’NP1’/>
<lemma s=’44’ e=’44’ lem=’.’ num=’11’ wnum=’11’ pos=’.’/>
</lemma-list>
</sentence>
<end-s n="44"/>
</p>
</rasp>
d.3.9. example.xml.3tag
<?xml version="1.0"?>
<rasp>
<p>We <e t="RP 0rp"><i>can not</i><c>cannot</c></e> stomach
<e t="RP 0rp"><i>an other</i><c>another</c></e>
<e t="RP 2rp"><i>christmastime</i><c>Christmastime</c></e> celebration with all
<e t="AGN 0agn"><i>kind</i><c>kinds</c></e> of
<e t="IN 2in"><i>delicacys</i><c>delicacies</c></e> for sweet
<e t="IN 2in"><i>tooths</i><c>teeth</c></e>.</p>
<p><e t="ID 0id"><i>In the other</i><c>On the other</c></e> hand, it would
<e t="W 0w"><i>be also</i><c>also be</c></e> a sham too disappoint
<e t="FD 3fd"><i>an</i><c>a</c></e> <e t="DJ 0dj"><i>hopefull</i><c>hopeful</c></e>
<e t="S 2s"><i>childly</i><c>(¿?)</c></e> heart.</p>
<p>The crème brûlée <e t="IV 0iv"><i>costed</i><c>cost</c></e> £18 at Mangiare &amp; Bere.</p>
</rasp>
150
appendix d. xml listings
d.3.10. example.xml.5
<?xml version="1.0"?>
<rasp>
<p>We cannot stomach another Christmastime celebration with all kinds of delicacies for sweet
teeth.</p>
<p>On the other hand, it would also be a sham too disappoint a hopeful (¿?) heart.</p>
<p>The crème brûlée cost £18 at Mangiare &amp; Bere.</p>
</rasp>
d.3.11. example.xml.5rasp
<?xml version="1.0"?>
<rasp>
<p>
<start-s n="0"/>
<sentence num=’1’>
<lemma-list>
<lemma s=’0’ e=’1’ lem=’We’ num=’1’ wnum=’1’ pos=’PPIS2’/>
<lemma s=’3’ e=’8’ lem=’cannot’ num=’2’ wnum=’2’ pos=’VM’/>
<lemma s=’10’ e=’16’ lem=’stomach’ num=’3’ wnum=’3’ pos=’VV0’/>
<lemma s=’18’ e=’24’ lem=’another’ num=’4’ wnum=’4’ pos=’DD1’/>
<lemma s=’26’ e=’38’ lem=’Christmastime’ num=’5’ wnum=’5’ pos=’NN1’/>
<lemma s=’40’ e=’50’ lem=’celebration’ num=’6’ wnum=’6’ pos=’NN1’/>
<lemma s=’52’ e=’55’ lem=’with’ num=’7’ wnum=’7’ pos=’IW’/>
<lemma s=’57’ e=’59’ lem=’all’ num=’8’ wnum=’8’ pos=’DB2’/>
<lemma s=’61’ e=’65’ lem=’kind’ affix=’s’ num=’9’ wnum=’9’ pos=’NN2’/>
<lemma s=’67’ e=’68’ lem=’of’ num=’10’ wnum=’10’ pos=’IO’/>
<lemma s=’70’ e=’79’ lem=’delicacy’ affix=’s’ num=’11’ wnum=’11’ pos=’NN2’/>
<lemma s=’81’ e=’83’ lem=’for’ num=’12’ wnum=’12’ pos=’IF’/>
<lemma s=’85’ e=’89’ lem=’sweet’ num=’13’ wnum=’13’ pos=’JJ’/>
<lemma s=’91’ e=’95’ lem=’tooth’ affix=’s’ num=’14’ wnum=’14’ pos=’NN2’/>
<lemma s=’96’ e=’96’ lem=’.’ num=’15’ wnum=’15’ pos=’.’/>
</lemma-list>
<nbest-parses num=’1’>
<parse-set pnum=’1’ score=’-19.371’>
<gr-list>
<gr type=’ncsubj’ head=’3’ dep=’1’></gr>
<gr type=’aux’ head=’3’ dep=’2’></gr>
<gr type=’iobj’ head=’3’ dep=’7’></gr>
<gr type=’dobj’ head=’3’ dep=’6’></gr>
<gr type=’dobj’ head=’7’ dep=’9’></gr>
<gr type=’iobj’ head=’9’ dep=’10’></gr>
<gr type=’dobj’ head=’10’ dep=’11’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’11’ dep=’12’></gr>
<gr type=’dobj’ head=’12’ dep=’14’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’14’ dep=’13’></gr>
<gr type=’ncmod’ subtype=’part’ head=’9’ dep=’8’></gr>
d.3. from chapter 7
151
<gr type=’det’ head=’6’ dep=’4’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’6’ dep=’5’></gr>
</gr-list>
</parse-set>
</nbest-parses>
</sentence>
<end-s n="96"/>
</p>
<p>
<start-s n="0"/>
<sentence num=’2’>
<lemma-list>
<lemma s=’0’ e=’1’ lem=’On’ num=’1’ wnum=’1’ pos=’II’/>
<lemma s=’3’ e=’5’ lem=’the’ num=’2’ wnum=’2’ pos=’AT’/>
<lemma s=’7’ e=’11’ lem=’other’ num=’3’ wnum=’3’ pos=’JB’/>
<lemma s=’13’ e=’16’ lem=’hand’ num=’4’ wnum=’4’ pos=’NN1’/>
<lemma s=’17’ e=’17’ lem=’,’ num=’5’ wnum=’5’ pos=’,’/>
<lemma s=’19’ e=’20’ lem=’it’ num=’6’ wnum=’6’ pos=’PPH1’/>
<lemma s=’22’ e=’26’ lem=’would’ num=’7’ wnum=’7’ pos=’VM’/>
<lemma s=’28’ e=’31’ lem=’also’ num=’8’ wnum=’8’ pos=’RR’/>
<lemma s=’33’ e=’34’ lem=’be’ num=’9’ wnum=’9’ pos=’VB0’/>
<lemma s=’36’ e=’36’ lem=’a’ num=’10’ wnum=’10’ pos=’AT1’/>
<lemma s=’38’ e=’41’ lem=’sham’ num=’11’ wnum=’11’ pos=’NN1’/>
<lemma s=’43’ e=’45’ lem=’too’ num=’12’ wnum=’12’ pos=’RG’/>
<lemma s=’47’ e=’56’ lem=’disappoint’ num=’13’ wnum=’13’ pos=’VV0’/>
<lemma s=’58’ e=’58’ lem=’a’ num=’14’ wnum=’14’ pos=’AT1’/>
<lemma s=’60’ e=’66’ lem=’hopeful’ num=’15’ wnum=’15’ pos=’JJ’/>
<lemma s=’68’ e=’71’ lem=’(¿?)’ num=’16’ wnum=’16’ pos=’NN1’/>
<lemma s=’73’ e=’77’ lem=’heart’ num=’17’ wnum=’17’ pos=’NN1’/>
<lemma s=’78’ e=’78’ lem=’.’ num=’18’ wnum=’18’ pos=’.’/>
</lemma-list>
<nbest-parses num=’1’>
<parse-set pnum=’1’ score=’-20.003’>
<gr-list>
<gr type=’ncmod’ subtype=’ ’ head=’9’ dep=’1’></gr>
<gr type=’ncsubj’ head=’9’ dep=’6’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’9’ dep=’8’></gr>
<gr type=’aux’ head=’9’ dep=’7’></gr>
<gr type=’ccomp’ subtype=’ ’ head=’9’ dep=’13’></gr>
<gr type=’ncsubj’ head=’13’ dep=’11’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’13’ dep=’12’></gr>
<gr type=’dobj’ head=’13’ dep=’17’></gr>
<gr type=’det’ head=’17’ dep=’14’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’17’ dep=’15’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’17’ dep=’16’></gr>
<gr type=’det’ head=’11’ dep=’10’></gr>
<gr type=’dobj’ head=’1’ dep=’4’></gr>
<gr type=’det’ head=’4’ dep=’2’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’4’ dep=’3’></gr>
152
appendix d. xml listings
</gr-list>
</parse-set>
</nbest-parses>
</sentence>
<end-s n="78"/>
</p>
<p>
<start-s n="0"/>
<sentence num=’3’>
<lemma-list>
<lemma s=’0’ e=’2’ lem=’The’ num=’1’ wnum=’1’ pos=’AT’/>
<lemma s=’4’ e=’8’ lem=’crème’ num=’2’ wnum=’2’ pos=’NN1’/>
<lemma s=’10’ e=’15’ lem=’brûlée’ num=’3’ wnum=’3’ pos=’NN1’/>
<lemma s=’17’ e=’20’ lem=’cost’ num=’4’ wnum=’4’ pos=’VVD’/>
<lemma s=’22’ e=’22’ lem=’£’ num=’5’ wnum=’5’ pos=’NNU’/>
<lemma s=’23’ e=’24’ lem=’18’ num=’6’ wnum=’6’ pos=’MC’/>
<lemma s=’26’ e=’27’ lem=’at’ num=’7’ wnum=’7’ pos=’II’/>
<lemma s=’29’ e=’36’ lem=’Mangiare’ num=’8’ wnum=’8’ pos=’NP1’/>
<lemma s=’38’ e=’38’ lem=’&amp;amp;’ num=’9’ wnum=’9’ pos=’CC’/>
<lemma s=’40’ e=’43’ lem=’Bere’ num=’10’ wnum=’10’ pos=’NP1’/>
<lemma s=’44’ e=’44’ lem=’.’ num=’11’ wnum=’11’ pos=’.’/>
</lemma-list>
<nbest-parses num=’1’>
<parse-set pnum=’1’ score=’-19.326’>
<gr-list>
<gr type=’ncsubj’ head=’4’ dep=’3’></gr>
<gr type=’iobj’ head=’4’ dep=’7’></gr>
<gr type=’dobj’ head=’4’ dep=’5’></gr>
<gr type=’dobj’ head=’7’ dep=’9’></gr>
<gr type=’conj’ head=’9’ dep=’8’></gr>
<gr type=’conj’ head=’9’ dep=’10’></gr>
<gr type=’ncmod’ subtype=’num’ head=’5’ dep=’6’></gr>
<gr type=’det’ head=’3’ dep=’1’></gr>
<gr type=’ncmod’ subtype=’ ’ head=’3’ dep=’2’></gr>
</gr-list>
</parse-set>
</nbest-parses>
</sentence>
<end-s n="44"/>
</p>
</rasp>
d.3.12. example.xml.pre/KET-Spanish-1865831.xml
<head url="1865831">
<candidate>
<exam>
<exam code>0085</exam code>
d.3. from chapter 7
153
<exam desc>Key English Test</exam desc>
<exam level>KET</exam level>
</exam>
<personnel>
<language>Spanish</language>
...
</personnel>
<scores and grades>
...
</scores and grades>
</candidate>
<text>
<answer1>
<question number>9</question number>
<exam score>4</exam score>
<original answer>
<p>We <e t="RP 0rp"><i>can not</i><c>cannot</c></e> stomach
<e t="RP 0rp"><i>an other</i><c>another</c></e>
<e t="RP 2rp"><i>christmastime</i><c>Christmastime</c></e>
celebration with all
<e t="AGN 0agn"><i>kind</i><c>kinds</c></e> of
<e t="IN 2in"><i>delicacys</i><c>delicacies</c></e> for sweet
<e t="IN 2in"><i>tooths</i><c>teeth</c></e>.</p>
<p><e t="ID 0id"><i>In the other</i><c>On the other</c></e>
hand, it would <e t="W 0w"><i>be also</i><c>also be</c></e>
a sham too disappoint <e t="FD 3fd"><i>an</i><c>a</c></e>
<e t="DJ 0dj"><i>hopefull</i><c>hopeful</c></e>
<e t="S 2s"><i>childly</i><c>(¿?)</c></e> heart.</p>
</original answer>
</answer1>
</text>
</head>
d.3.13. example.xml.pre/FCE-Italian-2845832.xml
<head url="2845832">
<candidate>
<exam>
<exam code>0100</exam code>
<exam desc>First Certificate of English</exam desc>
<exam level>FCE</exam level>
</exam>
<personnel>
<language>Italian</language>
...
</personnel>
<scores and grades>
...
154
appendix d. xml listings
</scores and grades>
</candidate>
<text>
<answer1>
<question number>7</question number>
<exam score>5</exam score>
<original answer>
<p>The crème brûlée <e t="IV 0iv"><i>costed</i><c>cost</c></e>
£18 at Mangiare &amp; Bere.</p>
</original answer>
</answer1>
</text>
</head>
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