Domain-Independent Abstract Generation for Focused Meeting Summarization

Domain-Independent Abstract Generation for Focused Meeting Summarization
Domain-Independent Abstract Generation
for Focused Meeting Summarization
Claire Cardie
Department of Computer Science
Cornell University
Ithaca, NY 14853
[email protected]
Lu Wang
Department of Computer Science
Cornell University
Ithaca, NY 14853
[email protected]
C: Looking at what we’ve got, we we want an LCD display with a spinning wheel.
B: You have to have some push-buttons, don’t you?
C: Just spinning and not scrolling, I would say.
B: I think the spinning wheel is definitely very now.
A: but since LCDs seems to be uh a definite yes,
C: We’re having push-buttons on the outside
C: and then on the inside an LCD with spinning wheel,
Abstract
We address the challenge of generating natural language abstractive summaries for spoken
meetings in a domain-independent fashion.
We apply Multiple-Sequence Alignment to induce abstract generation templates that can be
used for different domains. An Overgenerateand-Rank strategy is utilized to produce and
rank candidate abstracts. Experiments using in-domain and out-of-domain training on
disparate corpora show that our system uniformly outperforms state-of-the-art supervised
extract-based approaches. In addition, human
judges rate our system summaries significantly
higher than compared systems in fluency and
overall quality.
1
Decision Abstract (Summary):
The remote will have push buttons outside, and an LCD
and spinning wheel inside.
A: and um I’m not sure about the buttons being in the
shape of fruit though.
D: Maybe make it like fruity colours or something.
C: The power button could be like a big apple or something.
D: Um like I’m just thinking bright colours.
Problem Abstract (Summary):
How to incorporate a fruit and vegetable theme into the
remote.
Introduction
Meetings are a common way to collaborate,
share information and exchange opinions. Consequently, automatically generated meeting summaries could be of great value to people and businesses alike by providing quick access to the essential content of past meetings. Focused meeting summaries have been proposed as particularly
useful; in contrast to summaries of a meeting as
a whole, they refer to summaries of a specific aspect of a meeting, such as the DECISIONS reached,
PROBLEMS discussed, PROGRESS made or AC TION ITEMS that emerged (Carenini et al., 2011).
Our goal is to provide an automatic summarization system that can generate abstract-style focused meeting summaries to help users digest the
vast amount of meeting content in an easy manner.
Existing meeting summarization systems remain largely extractive: their summaries are comprised exclusively of patchworks of utterances selected directly from the meetings to be summarized (Riedhammer et al., 2010; Bui et al., 2009;
Xie et al., 2008). Although relatively easy to construct, extractive approaches fall short of producing concise and readable summaries, largely due
Figure 1: Clips from the AMI meeting corpus (Mc-
cowan et al., 2005). A, B, C and D refer to distinct
speakers. Also shown is the gold-standard (manual)
abstract (summary) for the decision and the problem.
to the noisy, fragmented, ungrammatical and unstructured text of meeting transcripts (Murray et
al., 2010b; Liu and Liu, 2009).
In contrast, human-written meeting summaries
are typically in the form of abstracts — distillations of the original conversation written in new
language. A user study from Murray et al. (2010b)
showed that people demonstrate a strong preference for abstractive summaries over extracts when
the text to be summarized is conversational. Consider, for example, the two types of focused summary along with their associated dialogue snippets
in Figure 1. We can see that extracts are likely to
include unnecessary and noisy information from
the meeting transcripts. On the contrary, the manually composed summaries (abstracts) are more
compact and readable, and are written in a distinctly non-conversational style.
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Proceedings of the 51st Annual Meeting of the Association for Computational Linguistics, pages 1395–1405,
c
Sofia, Bulgaria, August 4-9 2013. 2013
Association for Computational Linguistics
To address the limitations of extract-based summaries, we propose a complete and fully automatic
domain-independent abstract generation framework for focused meeting summarization. Following existing language generation research (Angeli et al., 2010; Konstas and Lapata, 2012), we
first perform content selection: given the dialogue acts relevant to one element of the meeting (e.g. a single decision or problem), we train
a classifier to identify summary-worthy phrases.
Next, we develop an “overgenerate-and-rank”
strategy (Walker et al., 2001; Heilman and Smith,
2010) for surface realization, which generates and
ranks candidate sentences for the abstract. After redundancy reduction, the full meeting abstract
can thus comprise the focused summary for each
meeting element. As described in subsequent sections, the generation framework allows us to identify and reformulate the important information for
the focused summary. Our contributions are as follows:
• To the best of our knowledge, our system is
the first fully automatic system to generate
natural language abstracts for spoken meetings.
• We present a novel template extraction algorithm, based on Multiple Sequence Alignment (MSA) (Durbin et al., 1998), to induce
domain-independent templates that guide abstract generation. MSA is commonly used
in bioinformatics to identify equivalent fragments of DNAs (Durbin et al., 1998) and
has also been employed for learning paraphrases (Barzilay and Lee, 2003).
• Although our framework requires labeled
training data for each type of focused summary (decisions, problems, etc.), we also
make initial tries for domain adaptation so
that our summarization method does not need
human-written abstracts for each new meeting domain (e.g. faculty meetings, theater
group meetings, project group meetings).
We instantiate the abstract generation framework on two corpora from disparate domains
— the AMI Meeting Corpus (Mccowan et al.,
2005) and ICSI Meeting Corpus (Janin et al.,
2003) — and produce systems to generate focused summaries with regard to four types of
meeting elements: D ECISIONs, P ROBLEMs, AC TION I TEMS s, and P ROGRESS . Automatic evaluation (using ROUGE (Lin and Hovy, 2003) and
BLEU (Papineni et al., 2002)) against manually
generated focused summaries shows that our summarizers uniformly and statistically significantly
outperform two baseline systems as well as a
state-of-the-art supervised extraction-based system. Human evaluation also indicates that the
abstractive summaries produced by our systems
are more linguistically appealing than those of
the utterance-level extraction-based system, preferring them over summaries from the extractionbased system of comparable semantic correctness
(62.3% vs. 37.7%).
Finally, we examine the generality of our model
across domains for two types of focused summarization — decisions and problems — by training the summarizer on out-of-domain data (i.e. the
AMI corpus for use on the ICSI meeting data,
and vice versa). The resulting systems yield results comparable to those from the same system
trained on in-domain data, and statistically significantly outperform supervised extractive summarization approaches trained on in-domain data.
2
Related Work
Most research on spoken dialogue summarization attempts to generate summaries for full dialogues (Carenini et al., 2011). Only recently has
the task of focused summarization been studied.
Supervised methods are investigated to identify
key phrases or utterances for inclusion in the decision summary (Fernández et al., 2008; Bui et
al., 2009). Based on Fernández et al. (2008), a
relation representation is proposed by Wang and
Cardie (2012) to form structured summaries; we
adopt this representation here for content selection.
Our research is also in line with generating abstractive summaries for conversations. Extractive approaches (Murray et al., 2005; Xie et al.,
2008; Galley, 2006) have been investigated extensively in conversation summarization. Murray et
al. (2010a) present an abstraction system consisting of interpretation and transformation steps. Utterances are mapped to a simple conversation ontology in the interpretation step according to their
type, such as a decision or problem. Then an integer linear programming approach is employed
to select the utterances that cover more entities as
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Dialogue Acts:
C: Looking at what we've got,
we we want [an LCD display
with a spinning wheel].
B: You have to have some
push-buttons, don't you?
C: Just spinning and not
scrolling , I would say .
B: I think the spinning wheel is
definitely very now.
A: but since LCDs seems to be
uh a definite yes,
C: We're having push-buttons
[on the outside]
C: and then on the inside an
LCD with spinning wheel,
Learned Templates
Relation
Extraction Relation Instances:
<want, an LCD display with a spinning
wheel>
<an LCD display, with a spinning
wheel>
<have, some push-buttons>
<having, push-buttons on the outside>
<push-buttons, on the outside>
<an LCD, with spinning wheel>
… (other possibilities)
<want, an LCD display with a spinning wheel>
One-Best
• The team will want an LCD display with a
Abstract:
spinning wheel.
The group decide to
• The team with work with an LCD display
use an LCD display
with a spinning wheel.
Template
Statistical with a spinning
• The group decide to use an LCD display with
wheel.
Filling
Ranking
a spinning wheel.
… (other possibilities)
<push-buttons, on the outside>
• Push-buttons are going to be on the outside.
• Push-buttons on the outside will be used.
• There will be push-buttons on the outside.
… (other possibilities)
… (all possible abstracts per relation
instance)
Content Selection
One-Best
Abstract:
Final Summary:
PostSelection The group decide to
use an LCD display with
a spinning wheel.
There will be pushbuttons on the outside.
There will be pushbuttons on the
outside.
… (one-best abstract
per relation instance)
Surface Realization
Figure 2: The abstract generation framework. It takes as input a cluster of meeting-item-specific dialogue acts,
from which one focused summary is constructed. Sample relation instances are denoted in bold (The indicators
are further italicized and the arguments are in [brackets]). Summary-worthy relation instances are identified by
content selection module (see Section 4) and then filled into the learned templates individually. A statistical ranker
subsequently selects one best abstract per relation instance (see Section 5.2). The post-selection component reduces
the redundancy and outputs the final summary (see Section 5.3).
determined by an external ontology. Liu and Liu
(2009) apply sentence compression on extracted
summary utterances. Though some of the unnecessary words are dropped, the resulting compressions can still be ungrammatical and unstructured.
This work is also broadly related to expert system-based language generation (Reiter
and Dale, 2000) and concept-to-text generation
tasks (Angeli et al., 2010; Konstas and Lapata,
2012), where the generation process is decomposed into content selection (or text planning) and
surface realization. For instance, Angeli et al.
(2010) learn from structured database records and
parallel textual descriptions. They generate texts
based on a series of decisions made to select the
records, fields, and proper templates for rendering. Those techniques that are tailored to specific
domains (e.g. weather forecasts or sportcastings)
cannot be directly applied to the conversational
data, as their input is well-structured and the templates learned are domain-specific.
3
Framework
Our domain-independent abstract generation
framework produces a summarizer that generates a grammatical abstract from a cluster of
meeting-element-related dialogue acts (DAs) —
all utterances associated with a single decision,
problem, action item or progress step of interest.
Note that identifying these DA clusters is a difficult task in itself (Bui et al., 2009). Accordingly,
our experiments evaluate two conditions — one
in which we assume that they are perfectly identified, and one in which we identify the clusters
automatically.
The summarizer consists of two major components and is depicted in Figure 2. Given the DA
cluster to be summarized, the Content Selection
module identifies a set of summary-worthy relation instances represented as indicator-argument
pairs (i.e. these constitute a finer-grained representation than DAs). The Surface Realization component then generates a short summary in three steps.
In the first step, each relation instance is filled into
templates with disparate structures that are learned
automatically from the training set (Template Filling). A statistical ranker then selects one best abstract per relation instance (Statistical Ranking).
Finally, selected abstracts are processed for redundancy removal in Post-Selection. Detailed descriptions for each individual step are provided in Sections 4 and 5.
4 Content Selection
Phrase-based content selection approaches have
been shown to support better meeting summaries (Fernández et al., 2008). Therefore, we
chose a content selection representation of a finer
granularity than an utterance: we identify relation
instances that can both effectively detect the crucial content and incorporate enough syntactic information to facilitate the downstream surface realization.
More specifically, our relation instances are
based on information extraction methods that
identify a lexical indicator (or trigger) that evokes
a relation of interest and then employ syntactic information, often in conjunction with semantic constraints, to find the argument constituent(or target phrase) to be extracted. Rela-
1397
Basic Features
number of words/content words
portion of content words/stopwords
number of content words in indicator/argument
number of content words that are also in previous DA
indicator/argument only contains stopword?
number of new nouns
Content Features
has capitalized word?
has proper noun?
TF/IDF/TFIDF min/max/average
Discourse Features
main speaker or not?
is in an adjacency pair (AP)?
is in the source/target of the AP?
number of source/target DA in the AP
is the target of the AP a positive/negative/neutral response?
is the source of the AP a question?
Syntax Features
indicator/argument constituent tag
dependency relation of indicator and argument
tion instances, then, are represented by indicatorargument pairs (Chen et al., 2011). For example,
in the DA cluster of Figure 2, hwant, an LCD display with a spinning wheeli and hpush-buttons, on
the outsidei are two relation instances.
Relation Instance Extraction We adopt and
extend the syntactic constraints from Wang and
Cardie (2012) to identify all relation instances in
the input utterances; the summary-worthy ones
will be selected by a discriminative classifier.
Constituent and dependency parses are obtained
by the Stanford parser (Klein and Manning, 2003).
Both the indicator and argument take the form of
constituents in the parse tree. We restrict the eligible indicator to be a noun or verb; the eligible arguments is a noun phrase (NP), prepositional
phrase (PP) or adjectival phrase (ADJP). A valid
indicator-argument pair should have at least one
content word and satisfy one of the following constraints:
• When the indicator is a noun, the argument
has to be a modifier or complement of the indicator.
• When the indicator is a verb, the argument
has to be the subject or the object if it is an
NP, or a modifier or complement of the indicator if it is a PP/ADJP.
We view relation extraction as a binary classification problem rather than a clustering task (Chen
et al., 2011). All relation instances can be categorized as summary-worthy or not, but only the
summary-worthy ones are used for abstract generation. A discriminative classifier is trained for
this purpose based on Support Vector Machines
(SVMs) (Joachims, 1998) with an RBF kernel.
For training data construction, we consider a relation instance to be a positive example if it shares
any content word with its corresponding abstracts,
and a negative example otherwise. The features
used are shown in Table 1.
5
Surface Realization
In this section, we describe surface realization,
which renders the relation instances into natural
language abstracts. This process begins with template extraction (Section 5.1). Once the templates
are learned, the relation instances from Section 4
are filled into the templates to generate an abstract
(see Section 5.2). Redundancy handling is discussed in Section 5.3.
Table 1: Features for content selection. Most are
adapted from previous work (Galley, 2006; Xie et al.,
2008; Wang and Cardie, 2012). Every basic or content feature is concatenated with the constituent tags of
indicator and argument to compose a new one. Main
speakers include the most talkative speaker (who has
said the most words) and other speakers whose word
count is more than 20% of the most talkative one (Xie
et al., 2008). Adjacency pair (AP) (Galley, 2006) is
an important conversational analysis concept; each AP
consists of a source utterance and a target utterance produced by different speakers.
5.1
Template Extraction
Sentence Clustering. Template extraction starts
with clustering the sentences that constitute the
manually generated abstracts in the training data
according to their lexical and structural similarity.
From each cluster, multiple-sequence alignment
techniques are employed to capture the recurring
patterns.
Intuitively, desirable templates are those that
can be applied in different domains to generate
the same type of focused summary (e.g. decision
or problem summaries). We do not want sentences to be clustered only because they describe
the same domain-specific details (e.g. they are all
about “data collection”), which will lead to fragmented templates that are not reusable for new domains. We therefore replace all appearances of
dates, numbers, and proper names with generic labels. We also replace words that appear in both
the abstract and supporting dialogue acts by a label indicating its phrase type. For any noun phrase
with its head word abstracted, the whole phrase is
also replaced with “NP”.
1398
1) The group were not sure whether to [include]VP [a recharger for the remote]NP .
2) The group were not sure whether to use [plastic and rubber or titanium for the case]NP .
3) The group were not sure whether [the remote control]NP should include [functions for
controlling video]NP .
4) They were not sure how much [a recharger]NP would cost to make .
… (Other abstracts)
Generic Label Replacement + Clustering
1) The group were not sure whether to VP NP .
2) The group were not sure whether to use NP .
3) The group were not sure whether NP should include NP .
4) They were not sure how much NP would cost to make .
MSA
use
The
start
group
were
not
sure
They
how
whether
to
NP
should
much would
cost
VP
NP
end
include
to
make
Template Induction
Template Examples:
Fine T1: The group were not sure whether to SLOTVP NP . (1, 2)
Fine T2: The group were not sure whether NP SLOTVP SLOTVP NP . (3)
Fine T3: SLOTNP were not sure SLOTWHADJP SLOTWHADJP NP SLOTVP SLOTVP SLOTVP SLOTVP
SLOTVP . (4)
Coarse T1: SLOTNP SLOTNP were not sure SLOTSBAR SLOTVP SLOTVP SLOTNP . (1, 2)
Coarse T2: SLOTNP SLOTNP were not sure SLOTSBAR SLOTNP SLOTVP SLOTVP SLOTNP . (3)
Coarse T3: SLOTNP were not sure SLOTWHADJP SLOTWHADJP SLOTNP SLOTVP SLOTVP SLOTVP
SLOTVP . (4)
Figure 3: Example of template extraction by MultipleSequence Alignment for problem abstracts from AMI.
Backbone nodes shared by at least 50% sentences are
shaded. The grammatical errors exist in the original
abstracts.
Following Barzilay and Lee (2003), we approach the sentence clustering task by hierarchical
complete-link clustering with a similarity metric
based on word n-gram overlap (n = 1, 2, 3). Clusters with fewer than three abstracts are removed1 .
Learning the Templates via MSA. For learning the structural patterns among the abstracts,
Multiple-Sequence Alignment (MSA) is first computed for each cluster. MSA takes as input multiple sentences and one scoring function to measure
the similarity between any two words. For insertions or deletions, a gap cost is also added. MSA
can thus find the best way to align the sequences
with insertions or deletions in accordance with the
scorer. However, computing an optimal MSA is
NP-complete (Wang and Jiang, 1994), thus we
implement an approximate algorithm (Needleman
and Wunsch, 1970) that iteratively aligns two sequences each time and treats the resulting alignment as a new sequence2 . Figure 3 demonstrates
an MSA computed from a sample cluster of ab1
Clustering stops when the similarity between any pairwise clusters is below 5. This is applied to every type of summarization. We tune the parameter on a small held-out development set by manually evaluating the induced templates. No
significant change is observed within a small range.
2
We adopt the scoring function for MSA from Barzilay
and Lee (2003), where aligning two identical words scores
1, inserting a gap scores −0.01, and aligning two different
words scores −0.5.
stracts. The MSA is represented in the form of
word lattice, from which we can detect the structural similarities shared by the sentences.
To transform the resulting MSAs into templates,
we need to decide whether a word in the sentence
should be retained to comprise the template or abstracted. The backbone nodes in an MSA are identified as the ones shared by more than 50%3 of the
cluster’s sentences (shaded in gray in Figure 3).
We then create a F INE template for each sentence
by abstracting the non-backbone words, i.e. replacing each of those words with a generic token
(last step in Figure 3). We also create a C OARSE
template that only preserves the nodes shared by
all of the cluster’s sentences. By using the operations above, domain-independent patterns are
thus identified and domain-specific details are removed.
Note that we do not explicitly evaluate the quality of the learned templates, which would require
a significant amount of manual evaluation. Instead, they are evaluated extrinsically. We encode
the templates as features (Angeli et al., 2010) that
could be selected or ignored in the succeeding abstract ranking model.
5.2
Template Filling
An Overgenerate-and-Rank Approach. Since
filling the relation instances into templates of distinct structures may result in abstracts of varying quality, we rank the abstracts based on the
features of the template, the transformation conducted, and the generated abstract. This is realized
by the Overgenerate-and-Rank strategy (Walker et
al., 2001; Heilman and Smith, 2010). It takes as
input a set of relation instances (from the same
cluster) R = {hindi , argi i}N
i=1 that are produced
by content selection component, a set of templates
T = {tj }M
j=1 that are represented as parsing trees,
a transformation function F (described below),
and a statistical ranker S for ranking the generated
abstracts, for which we defer description later in
this Section.
For each hindi , argi i, the overgenerate-andrank approach fills it into each template in T by
applying F to generate all possible abstracts. Then
the ranker S selects the best abstract absi . Postselection is conducted on the abstracts {absi }N
i=1
to form the final summary.
3
See Barzilay and Lee (2003) for a detailed discussion
about the choice of 50% according to pigeonhole principle.
1399
The transformation function F models the
constituent-level transformations of relation instances and their mappings to the parse trees of
templates. With the intuition that people will reuse
the relation instances from the transcripts albeit
not necessarily in their original form to write the
abstracts, we consider three major types of mapping operations for the indicator or argument in the
source pair, namely, Full-Constituent Mapping,
Sub-Constituent Mapping, and Removal. FullConstituent Mapping denotes that a source constituent is mapped directly to a target constituent
of the template parse tree with the same tag. SubConstituent Mapping encodes more complex and
flexible transformations in that a sub-constituent
of the source is mapped to a target constituent
with the same tag. This operation applies when
the source has a tag of PP or ADJP, in which case
its sub-constituent, if any, with a tag of NP, VP or
ADJP can be mapped to the target constituent with
the same tag. For instance, an argument “with a
spinning wheel” (PP) can be mapped to an NP in a
template because it has a sub-constituent “a spinning wheel” (NP). Removal means a source is not
mapped to any constituent in the template.
Formally, F is defined as:
F (hindsrc , arg src i, t) =
tar K
{hindtran
, argktran , indtar
k
k , argk i}k=1
where hindsrc , arg src i ∈ R is a relation instance (source pair); t ∈ T is a template; indtran
k
and argktran is the transformed pair of indsrc and
tar
arg src ; indtar
k and argk are constituents in t, and
they compose one target pair for hindsrc , arg src i.
We require that indsrc and arg src are not removed
at the same time. Moreover, for valid indtar
k and
tar
argk , the words subsumed by them should be all
abstracted in the template, and they do not overlap
in the parse tree.
To obtain the realized abstract, we traverse the
parse tree of the filled template in pre-order. The
words subsumed by the leaf nodes are thus collected sequentially.
Learning a Statistical Ranker. We utilize a discriminative ranker based on Support Vector Regression (SVR) (Smola and Schölkopf, 2004) to
rank the generated abstracts. Given the training data that includes clusters of gold-standard
summary-worthy relation instances, associated abstracts they support, and the parallel templates for
each abstract, training samples for the ranker are
Basic Features
number of words in indsrc /argsrc
number of new nouns in indsrc /argsrc
indtran
/argtran
only has stopword?
k
k
number of new nouns in indtran
/argtran
k
k
Structure Features
src
src
constituent tag of ind /arg
constituent tag of indsrc with constituent tag of indtar
constituent tag of argsrc with constituent tag of argtar
transformation of indsrc /argsrc combined with constituent tag
dependency relation of indsrc and argsrc
dependency relation of indtar and argtar
above 2 features have same value?
Template Features
template type (fine/coarse)
realized template (e.g. “the group decided to”)
number of words in template
the template has verb?
Realization Features
realization has verb?
realization starts with verb?
realization has adjacent verbs/NPs?
indsrc precedes/succeeds argsrc ?
indtar precedes/succeeds argtar ?
above 2 features have same value?
Language Model Features
|previous 1/2 words)
log pLM (first word in indtran
k
log pLM (realization)
log pLM (first word in argtran
|previous 1/2 words)
k
log pLM (realization)/length
log pLM (next word | last 1/2 words in indtran
)
k
log pLM (next word | last 1/2 words in argtran
)
k
Table 2: Features for abstracts ranking. The language
model features are based on a 5-gram language model
trained on Gigaword (Graff, 2003) by SRILM (Stolcke,
2002).
constructed according to the transformation function F mentioned above. Each sample is represented as:
tar
(hindsrc , arg src i, hindtran
, argktran , indtar
k
k , argk i, t, a)
where hindsrc , arg src i is the source pair,
hindtran
, argktran i is the transformed pair,
k
tar
hindk , argktar i is the target pair in template t,
and a is the abstract parallel to t.
We first find hindtar,abs
, argktar,abs i, which
k
is the corresponding constituent pair of
tar
hindtar
Then we identify
k , argk i in a.
the summary-worthy words subsumed by
hindtran
, argktran i that also appear in a. If those
k
words are all subsumed by hindtar,abs
, argktar,abs i,
k
then it is considered to be a positive sample, and
a negative sample otherwise. Table 2 displays the
features used in abstract ranking.
5.3
Post-Selection: Redundancy Handling.
Post-selection aims to maximize the information
coverage and minimize the redundancy of the
summary. Given the generated abstracts A =
1400
Input : relation instances R = {hindi , argi i}N
i=1 ,
generated abstracts A = {absi }N
i=1 , objective
function f , cost function C
Output: final abstract G
G ← Φ (empty set);
U ← A;
while U 6= Φ do
i )−f (A,G)
abs ← arg maxabsi ∈U f (A,G∪abs
;
C(absi )
if f (A, G ∪ abs) − f (A, G) ≥ 0 then
G ← G ∪ abs;
end
U ← U \ abs;
end
Algorithm 1:
Greedy algorithm for postselection to generate the final summary.
{absi }N
i=1 , we use a greedy algorithm (Lin and
Bilmes, 2010) to select a subset A0 , where A0 ⊆ A,
to form the final summary. We define wij as
the unigram similarity between abstracts absi and
absj , C(absi ) as the number of words in absi . We
employ the following objective function:
f (A, G) =
P
absi ∈A\G
P
absj ∈G
wi,j , G ⊆ A
Algorithm 1 sequentially finds an abstract with
the greatest ratio of objective function gain to
length, and add it to the summary if the gain is
non-negative.
6
Experimental Setup
Corpora. Two disparate corpora are used for
evaluation. The AMI meeting corpus (Mccowan
et al., 2005) contains 139 scenario-driven meetings, where groups of four people participate in
a series of four meetings for a fictitious project of
designing remote control. The ICSI meeting corpus (Janin et al., 2003) consists of 75 naturally occurring meetings, each of them has 4 to 10 participants. Compared to the fabricated topics in
AMI, the conversations in ICSI tend to be specialized and technical, e.g. discussion about speech
and language technology. We use 57 meetings in
ICSI and 139 meetings in AMI that include a short
(usually one-sentence), manually constructed abstract summarizing each important output for every meeting. Decision and problem summaries are
annotated for both corpora. AMI has extra action item summaries, and ICSI has progress summaries. The set of dialogue acts that support each
abstract are annotated as such.
System Inputs. We consider two system input
settings. In the True Clusterings setting, we
use the annotations to create perfect partitions of
the DAs for input to the system; in the System
Figure 4:
Content selection evaluation by using
ROUGE-SU4 (multiplied by 100). SVM-DA and
SVM-T OKEN denotes for supervised extract-based
methods with SVMs on utterance- and token-level.
Summaries for decision, problem, action item, and
progress are generated and evaluated for AMI and ICSI
(with names in parentheses). X-axis shows the number
of meetings used for training.
Clusterings setting, we employ a hierarchical agglomerative clustering algorithm used for this task
in (Wang and Cardie, 2011). DAs are grouped according to a classifier trained beforehand.
Baselines and Comparisons. We compare our
system with (1) two unsupervised baselines, (2)
two supervised extractive approaches, and (3) an
oracle derived from the gold standard abstracts.
Baselines. As in Riedhammer et al. (2010), the
L ONGEST DA in each cluster is selected as the
summary. The second baseline picks the cluster prototype (i.e. the DA with the largest TFIDF similarity with the cluster centroid) as the
summary according to Wang and Cardie (2011).
Although it is possible that important content is
spread over multiple DAs, both baselines allow
us to determine summary quality when summaries
are restricted to a single utterance.
Supervised Learning. We also compare our
approach to two supervised extractive summarization methods — Support Vector Machines (Joachims, 1998) trained with the same fea-
1401
tures as our system (see Table 1) to identify the important DAs (no syntax features) (Xie et al., 2008;
Sandu et al., 2010) or tokens (Fernández et al.,
2008) to include into the summary4 .
Oracle. We compute an oracle consisting of the
words from the DA cluster that also appear in the
associated abstract to reflect the gap between the
best possible extracts and the human abstracts.
7
Results
Content Selection Evaluation. We first employ
ROUGE (Lin and Hovy, 2003) to evaluate the
content selection component with respect to the
human written abstracts. ROUGE computes the
ngram overlapping between the system summaries
with the reference summaries, and has been used
for both text and speech summarization (Dang,
2005; Xie et al., 2008). We report ROUGE-2 (R2) and ROUGE-SU4 (R-SU4) that are shown to
correlate with human evaluation reasonably well.
In AMI, four meetings of different functions are
carried out in each group5 . 35 meetings for “conceptual design” are randomly selected for testing.
For ICSI, we reserve 12 meetings for testing.
The R-SU4 scores for each system are displayed
in Figure 4 and show that our system uniformly
outperforms the baselines and supervised systems.
The learning curve of our system is relatively flat,
which means not many training meetings are required to reach a usable performance level.
Note that the ROUGE scores are relative low
when the reference summaries are human abstracts, even for evaluation among abstracts produced by different annotators (Dang, 2005). The
intrinsic difference of styles between dialogue and
human abstract further lowers the scores. But the
trend is still respected among the systems.
Abstract Generation Evaluation. To evaluate
the full abstract generation system, the BLEU
score (Papineni et al., 2002) (the precision of unigrams and bigrams with a brevity penalty) is computed with human abstracts as reference. BLEU
has a fairly good agreement with human judgement and has been used to evaluate a variety of
language generation systems (Angeli et al., 2010;
Konstas and Lapata, 2012).
4
We use SVMlight (Joachims, 1999) with RBF kernel by
default parameters for SVM-based classifiers and regressor.
5
The four types of meetings in AMI are: project kick-off
(35 meetings), functional design (35 meetings), conceptual
design (35 meetings), and detailed design (34 meetings).
Figure 5: Full abstract generation system evaluation
by using BLEU (multiplied by 100). SVM-DA denotes for supervised extractive methods with SVMs on
utterance-level.
We are not aware of any existing work generating abstractive summaries for conversations.
Therefore, we compare our full system against
a supervised utterance-level extractive method
based on SVMs along with the baselines. The
BLEU scores in Figure 5 show that our system improves the scores consistently over the baselines
and the SVM-based approach.
Domain Adaptation Evaluation. We further
examine our system in domain adaptation scenarios for decision and problem summarization,
where we train the system on AMI for use on ICSI,
and vice versa. Table 3 indicates that, with both
true clusterings and system clusterings, our system trained on out-of-domain data achieves comparable performance with the same system trained
on in-domain data. In most experiments, it also
significantly outperforms the baselines and the
extract-based approaches (p < 0.05).
Human Evaluation. We randomly select 15 decision and 15 problem DA clusters (true clusterings). We evaluate fluency (is the text grammatical?) and semantic correctness (does the
summary convey the gist of the DAs in the cluster?) for O UR S YSTEM trained on I N-domain data
1402
System (True Clusterings)
C ENTROID DA
L ONGEST DA
SVM-DA (I N )
SVM-DA (O UT )
O UR S YSTEM (I N )
O UR S YSTEM (O UT )
O RACLE
System (System Clusterings)
C ENTROID DA
L ONGEST DA
SVM-DA (I N )
SVM-DA (O UT )
O UR S YSTEM (I N )
O UR S YSTEM (O UT )
O RACLE
R-2
1.3
1.6
3.4
2.7
4.5
4.6
7.5
R-2
1.4
1.4
2.6
3.4
3.5
3.9
6.4
AMI Decision
R-SU4
BLEU
3.0
7.7
3.3
7.0
4.7
9.7
4.2
6.6
6.2
11.6
6.1
10.3
12.0
22.8
AMI Decision
R-SU4
BLEU
3.3
3.8
3.3
5.7
4.6
10.5
5.8
10.3
5.4
11.7
6.4
11.4
12.0
15.1
R-2
1.8
2.8
3.4
3.1
4.9
4.8
9.9
R-2
1.4
1.7
3.5
2.7
4.4
4.1
8.2
ICSI Decision
R-SU4
BLEU
3.5
3.8
4.7
6.5
4.5
5.7
4.2
4.6
7.1
10.0
6.4
7.8
14.9
20.2
ICSI Decision
R-SU4
BLEU
2.1
2.0
3.4
5.5
6.5
7.1
4.8
6.3
7.4
9.1
5.1
8.4
15.2
17.6
R-2
1.0
1.0
1.4
1.4
3.1
3.5
6.6
R-2
0.8
0.8
1.8
2.1
3.3
3.6
6.5
AMI Problem
R-SU4
BLEU
2.7
4.2
3.0
3.6
2.4
5.0
2.2
2.5
4.8
7.2
4.7
6.2
11.3
18.9
AMI Problem
R-SU4
BLEU
2.8
2.9
3.2
4.1
3.7
4.9
3.8
4.3
4.6
9.5
5.6
8.9
13.0
20.9
R-2
1.0
1.2
1.6
1.3
4.0
3.0
6.4
R-2
0.9
0.9
1.8
1.5
2.3
1.8
5.5
ICSI Problem
R-SU4
BLEU
2.3
2.8
3.4
4.6
3.4
3.4
3.0
4.6
5.9
6.0
5.5
5.3
12.6
13.0
ICSI Problem
R-SU4
BLEU
2.3
1.8
3.4
4.4
4.0
4.6
3.8
3.5
4.2
7.4
4.0
6.8
11.9
15.5
Table 3: Domain adaptation evaluation. Systems trained on out-of-domain data are denoted with “(O UT)”, otherwise with “(I N)”. ROUGE and BLEU scores are multiplied by 100. Our systems that statistically significantly
outperform all the other approaches (except O RACLE) are in bold (p < 0.05, paired t-test). The numbers in italics
show the significant improvement over the baselines by our systems.
System
O UR S YSTEM (I N )
O UR S YSTEM (O UT )
SVM-DA (I N )
Fluency
Mean
S.D.
3.67
0.85
3.58
0.90
3.36
0.84
Semantic
Mean
S.D.
3.27
1.03
3.25
1.16
3.44
1.26
Length
Decision Summary:
Human: The remote will have push buttons outside, and
an LCD and spinning wheel inside.
Our System (In): The group decide to use an LCD display with a spinning wheel. There will be push-buttons on
the outside.
Our System (Out): LCD display is going to be with a
spinning wheel. It is necessary having push-buttons on
the outside.
SVM-DA: Looking at what we’ve got, we we want an
LCD display with a spinning wheel. Just spinning and not
scrolling, I would say. I think the spinning wheel is definitely very now. We’re having push-buttons on the outside
Problem Summary:
Human: How to incorporate a fruit and vegetable theme
into the remote.
Our System (In): Whether to include the shape of fruit.
The team had to thinking bright colors.
Our System (Out): It is unclear that the buttons being in
the shape of fruit.
SVM-DA: and um Im not sure about the buttons being in
the shape of fruit though.
23.65
24.17
38.83
Table 4: Human evaluation results of Fluency and Semantic correctness for the generated abstracts. The ratings are on 1 (worst) to 5 (best) scale. The average
Length of the abstracts for each system is also listed.
and O UT-of-domain data, and for the utterancelevel extraction system (SVM-DA) trained on indomain data. Each cluster of DAs along with three
randomly ordered summaries are presented to the
judges. Five native speaking Ph.D. students (none
are authors) performed the task.
We carry out an one-way Analysis of Variance
which shows significant differences in score as a
function of system (p < 0.05, paired t-test). Results in Table 4 demonstrate that our system summaries are significantly more compact and fluent
than the extract-based method (p < 0.05) while
semantic correctness is comparable.
The judges also rank the three summaries in
terms of the overall quality in content, conciseness and grammaticality. An inter-rater agreement
of Fleiss’s κ = 0.45 (moderate agreement (Landis
and Koch, 1977)) was computed. Judges selected
our system as the best system in 62.3% scenarios
(IN - DOMAIN: 35.6%, OUT- OF - DOMAIN: 26.7%).
Sample summaries are exhibited in Figure 6.
8
Conclusion
We presented a domain-independent abstract generation framework for focused meeting summarization. Experimental results on two disparate
meeting corpora show that our system can uni-
Figure 6: Sample decision and problem summaries generated by various systems for examples
in Figure 1.
formly outperform the state-of-the-art supervised
extraction-based systems in both automatic and
manual evaluation. Our system also exhibits an
ability to train on out-of-domain data to generate
abstracts for a new target domain.
9
Acknowledgments
This work was supported in part by National Science Foundation Grant IIS-0968450 and a gift
from Boeing. We thank Moontae Lee, Myle Ott,
Yiye Ruan, Chenhao Tan, and the ACL reviewers
for valuable suggestions and advice on various aspects of this work.
1403
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