Types of Machine Learning Algorithms

Types of Machine Learning Algorithms
Types of Machine Learning Algorithms
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X3
Types of Machine Learning Algorithms
Taiwo Oladipupo Ayodele
University of Portsmouth
United Kingdom
1. Machine Learning: Algorithms Types
Machine learning algorithms are organized into taxonomy, based on the desired outcome of
the algorithm. Common algorithm types include:
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Supervised learning --- where the algorithm generates a function that maps inputs
to desired outputs. One standard formulation of the supervised learning task is the
classification problem: the learner is required to learn (to approximate the behavior
of) a function which maps a vector into one of several classes by looking at several
input-output examples of the function.
Unsupervised learning --- which models a set of inputs: labeled examples are not
available.
Semi-supervised learning --- which combines both labeled and unlabeled examples
to generate an appropriate function or classifier.
Reinforcement learning --- where the algorithm learns a policy of how to act given
an observation of the world. Every action has some impact in the environment, and
the environment provides feedback that guides the learning algorithm.
Transduction --- similar to supervised learning, but does not explicitly construct a
function: instead, tries to predict new outputs based on training inputs, training
outputs, and new inputs.
Learning to learn --- where the algorithm learns its own inductive bias based on
previous experience.
The performance and computational analysis of machine learning algorithms is a branch of
statistics known as computational learning theory.
Machine learning is about designing algorithms that allow a computer to learn. Learning is
not necessarily involves consciousness but learning is a matter of finding statistical
regularities or other patterns in the data. Thus, many machine learning algorithms will
barely resemble how human might approach a learning task. However, learning algorithms
can give insight into the relative difficulty of learning in different environments.
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1.1 Supervised Learning Approach
Supervised learning 1 is fairly common in classification problems because the goal is often to
get the computer to learn a classification system that we have created. Digit recognition,
once again, is a common example of classification learning. More generally, classification
learning is appropriate for any problem where deducing a classification is useful and the
classification is easy to determine. In some cases, it might not even be necessary to give predetermined classifications to every instance of a problem if the agent can work out the
classifications for itself. This would be an example of unsupervised learning in a
classification context.
Supervised learning 2 often leaves the probability for inputs undefined. This model is not
needed as long as the inputs are available, but if some of the input values are missing, it is
not possible to infer anything about the outputs. Unsupervised learning, all the observations
are assumed to be caused by latent variables, that is, the observations is assumed to be at the
end of the causal chain. Examples of supervised learning and unsupervised learning are
shown in the figure 1 below:
Fig. 1. Examples of Supervised and Unsupervised Learning
Supervised learning 3 is the most common technique for training neural networks and
decision trees. Both of these techniques are highly dependent on the information given by
the pre-determined classifications. In the case of neural networks, the classification is used
to determine the error of the network and then adjust the network to minimize it, and in
decision trees, the classifications are used to determine what attributes provide the most
information that can be used to solve the classification puzzle. We'll look at both of these in
more detail, but for now, it should be sufficient to know that both of these examples thrive
on having some "supervision" in the form of pre-determined classifications.
Inductive machine learning is the process of learning a set of rules from instances (examples
in a training set), or more generally speaking, creating a classifier that can
be used to generalize from new instances. The process of applying supervised ML to a realworld problem is described in Figure F. The first step is collecting the dataset. If a requisite
expert is available, then s/he could suggest which fields (attributes, features) are the most
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http://www.aihorizon.com/essays/generalai/supervised_unsupervised_machine_learning.htm
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http://www.cis.hut.fi/harri/thesis/valpola_thesis/node34.html
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http://www.aihorizon.com/essays/generalai/supervised_unsupervised_machine_learning.htm
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informative. If not, then the simplest method is that of “brute-force,” which means
measuring everything available in the hope that the right (informative, relevant) features
can be isolated. However, a dataset collected by the “brute-force” method is not directly
suitable for induction. It contains in most cases noise and missing feature values, and
therefore requires significant pre-processing according to Zhang et al (Zhang, 2002).
The second step is the data preparation and data pre-processing. Depending on the
circumstances, researchers have a number of methods to choose from to handle missing data
(Batista, 2003). Hodge et al (Hodge, 2004) , have recently introduced a survey of
contemporary techniques for outlier (noise) detection. These researchers have identified the
techniques’ advantages and disadvantages. Instance selection is not only used to handle
noise but to cope with the infeasibility of learning from very large datasets. Instance
selection in these datasets is an optimization problem that attempts to maintain the mining
quality while minimizing the sample size. It reduces data and enables a data mining
algorithm to function and work effectively with very large datasets. There is a variety of
procedures for sampling instances from a large dataset. See figure 2 below.
Feature subset selection is the process of identifying and removing as many irrelevant and
redundant features as possible (Yu, 2004) . This reduces the dimensionality of the data and
enables data mining algorithms to operate faster and more effectively. The fact that many
features depend on one another often unduly influences the accuracy of supervised ML
classification models. This problem can be addressed by constructing new features from the
basic feature set. This technique is called feature construction/transformation. These newly
generated features may lead to the creation of more concise and accurate classifiers. In
addition, the discovery of meaningful features contributes to better comprehensibility of the
produced classifier, and a better understanding of the learned concept.Speech recognition
using hidden Markov models and Bayesian networks relies on some elements of
supervision as well in order to adjust parameters to, as usual, minimize the error on the
given inputs.Notice something important here: in the classification problem, the goal of the
learning algorithm is to minimize the error with respect to the given inputs. These inputs,
often called the "training set", are the examples from which the agent tries to learn. But
learning the training set well is not necessarily the best thing to do. For instance, if I tried to
teach you exclusive-or, but only showed you combinations consisting of one true and one
false, but never both false or both true, you might learn the rule that the answer is always
true. Similarly, with machine learning algorithms, a common problem is over-fitting the
data and essentially memorizing the training set rather than learning a more general
classification technique. As you might imagine, not all training sets have the inputs
classified correctly. This can lead to problems if the algorithm used is powerful enough to
memorize even the apparently "special cases" that don't fit the more general principles. This,
too, can lead to over fitting, and it is a challenge to find algorithms that are both powerful
enough to learn complex functions and robust enough to produce generalisable results.
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Problem
Identification of
Data
Data Pre-Processing
Definition of training set
Algorithm
selection
Parameter
Tuning
Training
Evaluation with test
set
NO
OK
YES
Classifier
Fig. 2. Machine Learning Supervise Process
1.2 Unsupervised learning
Unsupervised learning 4 seems much harder: the goal is to have the computer learn how to
do something that we don't tell it how to do! There are actually two approaches to
unsupervised learning. The first approach is to teach the agent not by giving explicit
categorizations, but by using some sort of reward system to indicate success. Note that this
type of training will generally fit into the decision problem framework because the goal is
not to produce a classification but to make decisions that maximize rewards. This approach
nicely generalizes to the real world, where agents might be rewarded for doing certain
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actions and punished for doing others. Often, a form of reinforcement learning can be used
for unsupervised learning, where the agent bases its actions on the previous rewards and
punishments without necessarily even learning any information about the exact ways that
its actions affect the world. In a way, all of this information is unnecessary because by
learning a reward function, the agent simply knows what to do without any processing
because it knows the exact reward it expects to achieve for each action it could take. This can
be extremely beneficial in cases where calculating every possibility is very time consuming
(even if all of the transition probabilities between world states were known). On the other
hand, it can be very time consuming to learn by, essentially, trial and error. But this kind of
learning can be powerful because it assumes no pre-discovered classification of examples. In
some cases, for example, our classifications may not be the best possible. One striking
exmaple is that the conventional wisdom about the game of backgammon was turned on its
head when a series of computer programs (neuro-gammon and TD-gammon) that learned
through unsupervised learning became stronger than the best human chess players merely
by playing themselves over and over. These programs discovered some principles that
surprised the backgammon experts and performed better than backgammon programs
trained on pre-classified examples. A second type of unsupervised learning is called
clustering. In this type of learning, the goal is not to maximize a utility function, but simply
to find similarities in the training data. The assumption is often that the clusters discovered
will match reasonably well with an intuitive classification. For instance, clustering
individuals based on demographics might result in a clustering of the wealthy in one group
and the poor in another. Although the algorithm won't have names to assign to these
clusters, it can produce them and then use those clusters to assign new examples into one or
the other of the clusters. This is a data-driven approach that can work well when there is
sufficient data; for instance, social information filtering algorithms, such as those that
Amazon.com use to recommend books, are based on the principle of finding similar groups
of people and then assigning new users to groups. In some cases, such as with social
information filtering, the information about other members of a cluster (such as what books
they read) can be sufficient for the algorithm to produce meaningful results. In other cases, it
may be the case that the clusters are merely a useful tool for a human analyst.
Unfortunately, even unsupervised learning suffers from the problem of overfitting the
training data. There's no silver bullet to avoiding the problem because any algorithm that
can learn from its inputs needs to be quite powerful.
Unsupervised learning algorithms according to Ghahramani (Ghahramani, 2008) are
designed to extract structure from data samples. The quality of a structure is measured by a
cost function which is usually minimized to infer optimal parameters characterizing the
hidden structure in the data. Reliable and robust inference requires a guarantee that
extracted structures are typical for the data source, i.e., similar structures have to be
extracted from a second sample set of the same data source. Lack of robustness is known as
over fitting from the statistics and the machine learning literature. In this talk I characterize
the over fitting phenomenon for a class of histogram clustering models which play a
prominent role in information retrieval, linguistic and computer vision applications.
Learning algorithms with robustness to sample fluctuations are derived from large
deviation results and the maximum entropy principle for the learning process.
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Unsupervised learning has produced many successes, such as world-champion calibre
backgammon programs and even machines capable of driving cars! It can be a powerful
technique when there is an easy way to assign values to actions. Clustering can be useful
when there is enough data to form clusters (though this turns out to be difficult at times)
and especially when additional data about members of a cluster can be used to produce
further results due to dependencies in the data. Classification learning is powerful when the
classifications are known to be correct (for instance, when dealing with diseases, it's
generally straight-forward to determine the design after the fact by an autopsy), or when the
classifications are simply arbitrary things that we would like the computer to be able to
recognize for us. Classification learning is often necessary when the decisions made by the
algorithm will be required as input somewhere else. Otherwise, it wouldn't be easy for
whoever requires that input to figure out what it means. Both techniques can be valuable
and which one you choose should depend on the circumstances--what kind of problem is
being solved, how much time is allotted to solving it (supervised learning or clustering is
often faster than reinforcement learning techniques), and whether supervised learning is
even possible.
1.3 Algorithm Types
In the area of supervised learning which deals much with classification. These are the
algorithms types:
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Linear Classifiers
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Logical Regression
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Naïve Bayes Classifier
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Perceptron
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Support Vector Machine
Quadratic Classifiers
K-Means Clustering
Boosting
Decision Tree
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Random Forest
Neural networks
Bayesian Networks
Linear Classifiers: In machine learning, the goal of classification is to group items that have
similar feature values, into groups. Timothy et al (Timothy Jason Shepard, 1998) stated that
a linear classifier achieves this by making a classification decision based on the value of
the linear combination of the features. If the input feature vector to the classifier is
a real vector
, then the output score is
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Types of Machine Learning Algorithms
where
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is a real vector of weights and f is a function that converts the dot product of the
two vectors into the desired output. The weight vector
is learned from a set of labelled
training samples. Often f is a simple function that maps all values above a certain threshold
to the first class and all other values to the second class. A more complex f might give the
probability that an item belongs to a certain class.
For a two-class classification problem, one can visualize the operation of a linear classifier as
splitting a high-dimensional input space with a hyperplane: all points on one side of the
hyper plane are classified as "yes", while the others are classified as "no". A linear classifier is
often used in situations where the speed of classification is an issue, since it is often the
fastest classifier, especially when
is sparse. However, decision trees can be faster. Also,
linear classifiers often work very well when the number of dimensions in
is large, as
in document classification, where each element in
is typically the number of counts of a
word in a document (see document-term matrix). In such cases, the classifier should be wellregularized.
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Support Vector Machine: A Support Vector Machine as stated by Luis et al
(Luis Gonz, 2005) (SVM) performs classification by constructing an Ndimensional hyper plane that optimally separates the data into two
categories. SVM models are closely related to neural networks. In fact, a SVM
model using a sigmoid kernel function is equivalent to a twolayer, perceptron neural network.
Support Vector Machine (SVM) models are a close cousin to classical
multilayer perceptron neural networks. Using a kernel function, SVM’s are
an alternative training method for polynomial, radial basis function and
multi-layer perceptron classifiers in which the weights of the network are
found by solving a quadratic programming problem with linear constraints,
rather than by solving a non-convex, unconstrained minimization problem as
in standard neural network training.
In the parlance of SVM literature, a predictor variable is called an attribute,
and a transformed attribute that is used to define the hyper plane is called
a feature. The task of choosing the most suitable representation is known
as feature selection. A set of features that describes one case (i.e., a row of
predictor values) is called a vector. So the goal of SVM modelling is to find
the optimal hyper plane that separates clusters of vector in such a way that
cases with one category of the target variable are on one side of the plane and
cases with the other category are on the other size of the plane. The vectors
near the hyper plane are the support vectors. The figure below presents an
overview of the SVM process.
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A Two-Dimensional Example
Before considering N-dimensional hyper planes, let’s look at a simple 2-dimensional
example. Assume we wish to perform a classification, and our data has a categorical target
variable with two categories. Also assume that there are two predictor variables with
continuous values. If we plot the data points using the value of one predictor on the X axis
and the other on the Y axis we might end up with an image such as shown below. One
category of the target variable is represented by rectangles while the other category is
represented by ovals.
In this idealized example, the cases with one category are in the lower left corner and the
cases with the other category are in the upper right corner; the cases are completely
separated. The SVM analysis attempts to find a 1-dimensional hyper plane (i.e. a line) that
separates the cases based on their target categories. There are an infinite number of possible
lines; two candidate lines are shown above. The question is which line is better, and how do
we define the optimal line.
The dashed lines drawn parallel to the separating line mark the distance between the
dividing line and the closest vectors to the line. The distance between the dashed lines is
called the margin. The vectors (points) that constrain the width of the margin are the support
vectors. The following figure illustrates this.
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Types of Machine Learning Algorithms
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An SVM analysis (Luis Gonz, 2005) finds the line (or, in general, hyper plane) that is
oriented so that the margin between the support vectors is maximized. In the figure above,
the line in the right panel is superior to the line in the left panel.
If all analyses consisted of two-category target variables with two predictor variables, and
the cluster of points could be divided by a straight line, life would be easy. Unfortunately,
this is not generally the case, so SVM must deal with (a) more than two predictor variables,
(b) separating the points with non-linear curves, (c) handling the cases where clusters
cannot be completely separated, and (d) handling classifications with more than two
categories.
In this chapter, we shall explain three main machine learning techniques with their
examples and how they perform in reality. These are:
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K-Means Clustering
Neural Network
Self Organised Map
1.3.1 K-Means Clustering
The basic step of k-means clustering is uncomplicated. In the beginning we determine
number of cluster K and we assume the centre of these clusters. We can take any random
objects as the initial centre or the first K objects in sequence can also serve as the initial
centre. Then the K means algorithm will do the three steps below until convergence.
Iterate until stable (= no object move group):
1.
Determine the centre coordinate
2.
Determine the distance of each object to the centre
3.
Group the object based on minimum distance
The Figure 3 shows a K- means flow diagram
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Fig. 3. K-means iteration
K-means (Bishop C. M., 1995) and (Tapas Kanungo, 2002) is one of the simplest
unsupervised learning algorithms that solve the well known clustering problem. The
procedure follows a simple and easy way to classify a given data set through a certain
number of clusters (assume k clusters) fixed a priori. The main idea is to define k centroids,
one for each cluster. These centroids shoud be placed in a cunning way because of different
location causes different result. So, the better choice is to place them as much as possible far
away from each other. The next step is to take each point belonging to a given data set and
associate it to the nearest centroid. When no point is pending, the first step is completed and
an early groupage is done. At this point we need to re-calculate k new centroids as
barycenters of the clusters resulting from the previous step. After we have these k new
centroids, a new binding has to be done between the same data set points and the nearest
new centroid. A loop has been generated. As a result of this loop we may notice that the k
centroids change their location step by step until no more changes are done. In other words
centroids do not move any more.
Finally, this algorithm aims at minimizing an objective function, in this case a squared error
function. The objective function
,
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Types of Machine Learning Algorithms
where
is a chosen distance measure between a data point
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and the cluster
centre
, is an indicator of the distance of the n data points from their respective cluster
centres.
The algorithm in figure 4 is composed of the following steps:
1.
Place K points into the space represented by the objects that
are being clustered. These points represent initial group
centroids.
2.
Assign each object to the group that has the closest
centroid.
3.
When all objects have been assigned, recalculate the
positions of the K centroids.
4.
Repeat Steps 2 and 3 until the centroids no longer move.
This produces a separation of the objects into groups from
which the metric to be minimized can be calculated.
Although it can be proved that the procedure will always terminate, the k-means algorithm
does not necessarily find the most optimal configuration, corresponding to the global
objective function minimum. The algorithm is also significantly sensitive to the initial
randomly selected cluster centres. The k-means algorithm can be run multiple times to
reduce this effect. K-means is a simple algorithm that has been adapted to many problem
domains. As we are going to see, it is a good candidate for extension to work with fuzzy
feature vectors.
An example
Suppose that we have n sample feature vectors x1, x2, ..., xn all from the same class, and we
know that they fall into k compact clusters, k < n. Let mi be the mean of the vectors in cluster
i. If the clusters are well separated, we can use a minimum-distance classifier to separate
them. That is, we can say that x is in cluster i if || x - mi || is the minimum of all the k
distances. This suggests the following procedure for finding the k means:
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Make initial guesses for the means m1, m2, ..., mk
Until there are no changes in any mean
Use the estimated means to classify the samples into clusters
For i from 1 to k
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Replace mi with the mean of all of the samples for
cluster i
end_for
end_until
Here is an example showing how the means m1 and m2 move into the centers of two
clusters.
This is a simple version of the k-means procedure. It can be viewed as a greedy algorithm
for partitioning the n samples into k clusters so as to minimize the sum of the squared
distances to the cluster centers. It does have some weaknesses:
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The way to initialize the means was not specified. One popular way to start is to
randomly choose k of the samples.
The results produced depend on the initial values for the means, and it frequently
happens that suboptimal partitions are found. The standard solution is to try a
number of different starting points.
It can happen that the set of samples closest to mi is empty, so that mi cannot be
updated. This is an annoyance that must be handled in an implementation, but that
we shall ignore.
The results depend on the metric used to measure || x - mi ||. A popular solution
is to normalize each variable by its standard deviation, though this is not always
desirable.
The results depend on the value of k.
This last problem is particularly troublesome, since we often have no way of knowing how
many clusters exist. In the example shown above, the same algorithm applied to the same
data produces the following 3-means clustering. Is it better or worse than the 2-means
clustering?
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Unfortunately there is no general theoretical solution to find the optimal number of clusters
for any given data set. A simple approach is to compare the results of multiple runs with
different k classes and choose the best one according to a given criterion
1.3.2 Neural Network
Neural networks (Bishop C. M., 1995) can actually perform a number of regression
and/or classification tasks at once, although commonly each network performs only one. In
the vast majority of cases, therefore, the network will have a single output variable,
although in the case of many-state classification problems, this may correspond to a number
of output units (the post-processing stage takes care of the mapping from output units to
output variables). If you do define a single network with multiple output variables, it may
suffer from cross-talk (the hidden neurons experience difficulty learning, as they are
attempting to model at least two functions at once). The best solution is usually to train
separate networks for each output, then to combine them into an ensemble so that they can
be run as a unit. Neural methods are:
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Multilayer Perceptrons: This is perhaps the most popular network architecture in
use today, due originally to Rumelhart and McClelland (1986) and discussed at
length in most neural network textbooks (e.g., Bishop, 1995). This is the type of
network discussed briefly in previous sections: the units each perform a biased
weighted sum of their inputs and pass this activation level through a transfer
function to produce their output, and the units are arranged in a layered feed
forward topology. The network thus has a simple interpretation as a form of inputoutput model, with the weights and thresholds (biases) the free parameters of the
model. Such networks can model functions of almost arbitrary complexity, with
the number of layers, and the number of units in each layer, determining the
function complexity. Important issues in Multilayer Perceptrons (MLP) design
include specification of the number of hidden layers and the number of units in
these layers (Bishop C. M., 1995), (D. Michie, 1994).
The number of input and output units is defined by the problem (there may be
some uncertainty about precisely which inputs to use, a point to which we will
return later. However, for the moment we will assume that the input variables are
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intuitively selected and are all meaningful). The number of hidden units to use is
far from clear. As good a starting point as any is to use one hidden layer, with the
number of units equal to half the sum of the number of input and output units.
Again, we will discuss how to choose a sensible number later.
Training Multilayer Perceptrons: Once the number of layers, and number of units
in each layer, has been selected, the network's weights and thresholds must be set
so as to minimize the prediction error made by the network. This is the role of
the training algorithms. The historical cases that you have gathered are used to
automatically adjust the weights and thresholds in order to minimize this error.
This process is equivalent to fitting the model represented by the network to the
training data available. The error of a particular configuration of the network can
be determined by running all the training cases through the network, comparing
the actual output generated with the desired or target outputs. The differences are
combined together by an error function to give the network error. The most
common error functions are the sum squared error (used for regression problems),
where the individual errors of output units on each case are squared and summed
together, and the cross entropy functions (used for maximum likelihood
classification).
In traditional modeling approaches (e.g., linear modeling) it is possible to
algorithmically determine the model configuration that absolutely minimizes this
error. The price paid for the greater (non-linear) modeling power of neural
networks is that although we can adjust a network to lower its error, we can never
be sure that the error could not be lower still.
A helpful concept here is the error surface. Each of the N weights and thresholds of the
network (i.e., the free parameters of the model) is taken to be a dimension in space.
The N+1th dimension is the network error. For any possible configuration of weights the
error can be plotted in the N+1th dimension, forming an error surface. The objective of
network training is to find the lowest point in this many-dimensional surface.
In a linear model with sum squared error function, this error surface is a parabola (a
quadratic), which means that it is a smooth bowl-shape with a single minimum. It is
therefore "easy" to locate the minimum.
Neural network error surfaces are much more complex, and are characterized by a number
of unhelpful features, such as local minima (which are lower than the surrounding terrain,
but above the global minimum), flat-spots and plateaus, saddle-points, and long narrow
ravines.
It is not possible to analytically determine where the global minimum of the error surface is,
and so neural network training is essentially an exploration of the error surface. From an
initially random configuration of weights and thresholds (i.e., a random point on the error
surface), the training algorithms incrementally seek for the global minimum. Typically, the
gradient (slope) of the error surface is calculated at the current point, and used to make a
downhill move. Eventually, the algorithm stops in a low point, which may be a local
minimum (but hopefully is the global minimum).
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The Back Propagation Algorithm: The best-known example of a neural
network training algorithm is back propagation (Haykin, 19994), (Patterson,
19996), (Fausett, 19994). Modern second-order algorithms such as conjugate
gradient descent and Levenberg-Marquardt (see Bishop, 1995; Shepherd, 1997) (both
included in ST Neural Networks) are substantially faster (e.g., an order of
magnitude faster) for many problems, but back propagation still has advantages in
some circumstances, and is the easiest algorithm to understand. We will introduce
this now, and discuss the more advanced algorithms later. In back propagation, the
gradient vector of the error surface is calculated. This vector points along the line
of steepest descent from the current point, so we know that if we move along it a
"short" distance, we will decrease the error. A sequence of such moves (slowing as
we near the bottom) will eventually find a minimum of some sort. The difficult
part is to decide how large the steps should be.
Large steps may converge more quickly, but may also overstep the solution or (if
the error surface is very eccentric) go off in the wrong direction. A classic example
of this in neural network training is where the algorithm progresses very slowly
along a steep, narrow, valley, bouncing from one side across to the other. In
contrast, very small steps may go in the correct direction, but they also require a
large number of iterations. In practice, the step size is proportional to the slope (so
that the algorithm settles down in a minimum) and to a special constant:
the learning rate. The correct setting for the learning rate is application-dependent,
and is typically chosen by experiment; it may also be time-varying, getting smaller
as the algorithm progresses.
The algorithm is also usually modified by inclusion of a momentum term: this encourages
movement in a fixed direction, so that if several steps are taken in the same direction, the
algorithm "picks up speed", which gives it the ability to (sometimes) escape local minimum,
and also to move rapidly over flat spots and plateaus.
The algorithm therefore progresses iteratively, through a number of epochs. On each epoch,
the training cases are each submitted in turn to the network, and target and actual outputs
compared and the error calculated. This error, together with the error surface gradient, is
used to adjust the weights, and then the process repeats. The initial network configuration is
random, and training stops when a given number of epochs elapses, or when the error
reaches an acceptable level, or when the error stops improving (you can select which of
these stopping conditions to use).
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Over-learning and Generalization: One major problem with the approach
outlined above is that it doesn't actually minimize the error that we are really
interested in - which is the expected error the network will make when new cases
are submitted to it. In other words, the most desirable property of a network is its
ability to generalize to new cases. In reality, the network is trained to minimize the
error on the training set, and short of having a perfect and infinitely large training
set, this is not the same thing as minimizing the error on the real error surface - the
error surface of the underlying and unknown model (Bishop C. M., 1995).
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The most important manifestation of this distinction is the problem of over-learning,
or over-fitting. It is easiest to demonstrate this concept using polynomial curve fitting rather
than neural networks, but the concept is precisely the same.
A polynomial is an equation with terms containing only constants and powers of the
variables. For example:
y=2x+3
y=3x2+4x+1
Different polynomials have different shapes, with larger powers (and therefore larger
numbers of terms) having steadily more eccentric shapes. Given a set of data, we may want
to fit a polynomial curve (i.e., a model) to explain the data. The data is probably noisy, so we
don't necessarily expect the best model to pass exactly through all the points. A low-order
polynomial may not be sufficiently flexible to fit close to the points, whereas a high-order
polynomial is actually too flexible, fitting the data exactly by adopting a highly eccentric
shape that is actually unrelated to the underlying function. See figure 4 below.
Fig. 4. High-order polynomial sample
Neural networks have precisely the same problem. A network with more weights models a
more complex function, and is therefore prone to over-fitting. A network with less weight
may not be sufficiently powerful to model the underlying function. For example, a network
with no hidden layers actually models a simple linear function. How then can we select the
right complexity of network? A larger network will almost invariably achieve a lower error
eventually, but this may indicate over-fitting rather than good modeling.
The answer is to check progress against an independent data set, the selection set. Some of
the cases are reserved, and not actually used for training in the back propagation algorithm.
Instead, they are used to keep an independent check on the progress of the algorithm. It is
invariably the case that the initial performance of the network on training and selection sets
is the same (if it is not at least approximately the same, the division of cases between the two
sets is probably biased). As training progresses, the training error naturally drops, and
providing training is minimizing the true error function, the selection error drops too.
However, if the selection error stops dropping, or indeed starts to rise, this indicates that the
network is starting to overfit the data, and training should cease. When over-fitting occurs
during the training process like this, it is called over-learning. In this case, it is usually
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advisable to decrease the number of hidden units and/or hidden layers, as the network is
over-powerful for the problem at hand. In contrast, if the network is not sufficiently
powerful to model the underlying function, over-learning is not likely to occur, and neither
training nor selection errors will drop to a satisfactory level.
The problems associated with local minima, and decisions over the size of network to use,
imply that using a neural network typically involves experimenting with a large number of
different networks, probably training each one a number of times (to avoid being fooled by
local minima), and observing individual performances. The key guide to performance here
is the selection error. However, following the standard scientific precept that, all else being
equal, a simple model is always preferable to a complex model, you can also select a smaller
network in preference to a larger one with a negligible improvement in selection error.
A problem with this approach of repeated experimentation is that the selection set plays a
key role in selecting the model, which means that it is actually part of the training process.
Its reliability as an independent guide to performance of the model is therefore
compromised - with sufficient experiments, you may just hit upon a lucky network that
happens to perform well on the selection set. To add confidence in the performance of the
final model, it is therefore normal practice (at least where the volume of training data allows
it) to reserve a third set of cases - the test set. The final model is tested with the test set data,
to ensure that the results on the selection and training set are real, and not artifacts of the
training process. Of course, to fulfill this role properly the test set should be used only once if it is in turn used to adjust and reiterate the training process, it effectively becomes
selection data!
This division into multiple subsets is very unfortunate, given that we usually have less data
than we would ideally desire even for a single subset. We can get around this problem by
resampling. Experiments can be conducted using different divisions of the available data
into training, selection, and test sets. There are a number of approaches to this subset,
including random (monte-carlo) resampling, cross-validation, and bootstrap. If we make
design decisions, such as the best configuration of neural network to use, based upon a
number of experiments with different subset examples, the results will be much more
reliable. We can then either use those experiments solely to guide the decision as to which
network types to use, and train such networks from scratch with new samples (this removes
any sampling bias); or, we can retain the best networks found during the sampling process,
but average their results in an ensemble, which at least mitigates the sampling bias.
To summarize, network design (once the input variables have been selected) follows a
number of stages:
•
•
•
Select an initial configuration (typically, one hidden layer with the number of
hidden units set to half the sum of the number of input and output units).
Iteratively conduct a number of experiments with each configuration, retaining
the best network (in terms of selection error) found. A number of experiments are
required with each configuration to avoid being fooled if training locates a local
minimum, and it is also best to resample.
On each experiment, if under-learning occurs (the network doesn't achieve an
acceptable performance level) try adding more neurons to the hidden layer(s). If
this doesn't help, try adding an extra hidden layer.
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•
•
•
If over-learning occurs (selection error starts to rise) try removing hidden units
(and possibly layers).
Once you have experimentally determined an effective configuration for your
networks, resample and generate new networks with that configuration.
Data Selection: All the above stages rely on a key assumption. Specifically, the
training, verification and test data must be representative of the underlying model
(and, further, the three sets must be independently representative). The old
computer science adage "garbage in, garbage out" could not apply more strongly
than in neural modeling. If training data is not representative, then the model's
worth is at best compromised. At worst, it may be useless. It is worth spelling out
the kind of problems which can corrupt a training set:
The future is not the past. Training data is typically historical. If circumstances have
changed, relationships which held in the past may no longer hold. All eventualities must be
covered. A neural network can only learn from cases that are present. If people with
incomes over $100,000 per year are a bad credit risk, and your training data includes nobody
over $40,000 per year, you cannot expect it to make a correct decision when it encounters
one of the previously-unseen cases. Extrapolation is dangerous with any model, but some
types of neural network may make particularly poor predictions in such circumstances.
A network learns the easiest features it can. A classic (possibly apocryphal) illustration of
this is a vision project designed to automatically recognize tanks. A network is trained on a
hundred pictures including tanks, and a hundred not. It achieves a perfect 100% score.
When tested on new data, it proves hopeless. The reason? The pictures of tanks are taken on
dark, rainy days; the pictures without on sunny days. The network learns to distinguish the
(trivial matter of) differences in overall light intensity. To work, the network would need
training cases including all weather and lighting conditions under which it is expected to
operate - not to mention all types of terrain, angles of shot, distances...
Unbalanced data sets. Since a network minimizes an overall error, the proportion of types of
data in the set is critical. A network trained on a data set with 900 good cases and 100 bad
will bias its decision towards good cases, as this allows the algorithm to lower the overall
error (which is much more heavily influenced by the good cases). If the representation of
good and bad cases is different in the real population, the network's decisions may be
wrong. A good example would be disease diagnosis. Perhaps 90% of patients routinely
tested are clear of a disease. A network is trained on an available data set with a 90/10 split.
It is then used in diagnosis on patients complaining of specific problems, where the
likelihood of disease is 50/50. The network will react over-cautiously and fail to recognize
disease in some unhealthy patients. In contrast, if trained on the "complainants" data, and
then tested on "routine" data, the network may raise a high number of false positives. In
such circumstances, the data set may need to be crafted to take account of the distribution of
data (e.g., you could replicate the less numerous cases, or remove some of the numerous
cases), or the network's decisions modified by the inclusion of a loss matrix (Bishop C. M.,
1995). Often, the best approach is to ensure even representation of different cases, then to
interpret the network's decisions accordingly.
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1.3.3 Self Organised Map
Self Organizing Feature Map (SOFM, or Kohonen) networks are used quite differently to the
other networks. Whereas all the other networks are designed for supervised
learning tasks, SOFM networks are designed primarily for unsupervised learning (Haykin,
19994), (Patterson, 19996), (Fausett, 19994) (Whereas in supervised learning the training data
set contains cases featuring input variables together with the associated outputs (and the
network must infer a mapping from the inputs to the outputs), in unsupervised learning the
training data set contains only input variables. At first glance this may seem strange.
Without outputs, what can the network learn? The answer is that the SOFM
network attempts to learn the structure of the data.
Also Kohonen (Kohonen, 1997) explained one possible use is therefore in exploratory data
analysis. The SOFM network can learn to recognize clusters of data, and can also relate
similar classes to each other. The user can build up an understanding of the data, which is
used to refine the network. As classes of data are recognized, they can be labelled, so that
the network becomes capable of classification tasks. SOFM networks can also be used for
classification when output classes are immediately available - the advantage in this case is
their ability to highlight similarities between classes.
A second possible use is in novelty detection. SOFM networks can learn to recognize
clusters in the training data, and respond to it. If new data, unlike previous cases, is
encountered, the network fails to recognize it and this indicates novelty.
A SOFM network has only two layers: the input layer, and an output layer of radial units
(also known as the topological map layer). The units in the topological map layer are laid
out in space - typically in two dimensions (although ST Neural Networks also supports onedimensional Kohonen networks).
SOFM networks (Patterson, 19996) are trained using an iterative algorithm. Starting with an
initially-random set of radial centres, the algorithm gradually adjusts them to reflect the
clustering of the training data. At one level, this compares with the sub-sampling and KMeans algorithms used to assign centres in SOM network and indeed the SOFM algorithm
can be used to assign centres for these types of networks. However, the algorithm also acts
on a different level.
The iterative training procedure also arranges the network so that units representing centres
close together in the input space are also situated close together on the topological map. You
can think of the network's topological layer as a crude two-dimensional grid, which must be
folded and distorted into the N-dimensional input space, so as to preserve as far as possible
the original structure. Clearly any attempt to represent an N-dimensional space in two
dimensions will result in loss of detail; however, the technique can be worthwhile in
allowing the user to visualize data which might otherwise be impossible to understand.
The basic iterative Kohonen algorithm simply runs through a number of epochs, on each
epoch executing each training case and applying the following algorithm:
•
•
Select the winning neuron (the one who's centre is nearest to the input case);
Adjust the winning neuron to be more like the input case (a weighted sum of the
old neuron centre and the training case).
The algorithm uses a time-decaying learning rate, which is used to perform the weighted
sum and ensures that the alterations become more subtle as the epochs pass. This ensures
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that the centres settle down to a compromise representation of the cases which cause
that neuron to win. The topological ordering property is achieved by adding the concept of
a neighbourhood to the algorithm. The neighbourhood is a set of neurons surrounding the
winning neuron. The neighbourhood, like the learning rate, decays over time, so that
initially quite a large number of neurons belong to the neighbourhood (perhaps almost the
entire topological map); in the latter stages the neighbourhood will be zero (i.e., consists
solely of the winning neuron itself). In the Kohonen algorithm, the adjustment of neurons is
actually applied not just to the winning neuron, but to all the members of the current
neighbourhood.
The effect of this neighbourhood update is that initially quite large areas of the network are
"dragged towards" training cases - and dragged quite substantially. The network develops a
crude topological ordering, with similar cases activating clumps of neurons in
the topological map. As epochs pass the learning rate and neighbourhood both decrease, so
that finer distinctions within areas of the map can be drawn, ultimately resulting in finetuning of individual neurons. Often, training is deliberately conducted in two distinct
phases: a relatively short phase with high learning rates and neighbourhood, and a long
phase with low learning rate and zero or near-zero neighbourhoods.
Once the network has been trained to recognize structure in the data, it can be used as a
visualization tool to examine the data. The Win Frequencies Datasheet (counts of the number
of times each neuron wins when training cases are executed) can be examined to see if
distinct clusters have formed on the map. Individual cases are executed and the topological
map observed, to see if some meaning can be assigned to the clusters (this usually involves
referring back to the original application area, so that the relationship between clustered
cases can be established). Once clusters are identified, neurons in the topological map are
labelled to indicate their meaning (sometimes individual cases may be labelled, too). Once
the topological map has been built up in this way, new cases can be submitted to the
network. If the winning neuron has been labelled with a class name, the network can
perform classification. If not, the network is regarded as undecided.
SOFM networks also make use of the accept threshold, when performing classification.
Since the activation level of a neuron in a SOFM network is the distance of the neuron from
the input case, the accept threshold acts as a maximum recognized distance. If the activation
of the winning neuron is greater than this distance, the SOFM network is regarded as
undecided. Thus, by labelling all neurons and setting the accept threshold appropriately, a
SOFM network can act as a novelty detector (it reports undecided only if the input case is
sufficiently dissimilar to all radial units).
SOFM networks as expressed by Kohonen (Kohonen, 1997) are inspired by some known
properties of the brain. The cerebral cortex is actually a large flat sheet (about 0.5m squared;
it is folded up into the familiar convoluted shape only for convenience in fitting into the
skull!) with known topological properties (for example, the area corresponding to the hand
is next to the arm, and a distorted human frame can be topologically mapped out in two
dimensions on its surface).
1.4 Grouping Data Using Self Organise Map
The first part of a SOM is the data. Above are some examples of 3 dimensional data which
are commonly used when experimenting with SOMs. Here the colours are represented in
three dimensions (red, blue, and green.) The idea of the self-organizing maps is to project
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the n-dimensional data (here it would be colour and would be 3 dimensions) into something
that be better understood visually (in this case it would be a 2 dimensional image map).
Fig. 5. Sample Data
In this case one would expect the dark blue and the greys to end up near each other on a
good map and yellow close to both the red and the green. The second components to SOMs
are the weight vectors. Each weight vector has two components to them which I have here
attempted to show in the image below. The first part of a weight vector is its data. This is of
the same dimensions as the sample vectors and the second part of a weight vector is its
natural location. The good thing about colour is that the data can be shown by displaying
the color, so in this case the color is the data, and the location is the x,y position of the pixel
on the screen.
Fig. 6. 2D Array Weight of Vector
In this example, 2D array of weight vectors was used and would look like figure 5 above.
This picture is a skewed view of a grid where you have the n-dimensional array for each
weight and each weight has its own unique location in the grid. Weight vectors don’t
necessarily have to be arranged in 2 dimensions, a lot of work has been done using SOMs of
1 dimension, but the data part of the weight must be of the same dimensions as the sample
vectors.Weights are sometimes referred to as neurons since SOMs are actually neural
networks. SOM Algorithm. The way that SOMs go about organizing themselves is by
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competeting for representation of the samples. Neurons are also allowed to change
themselves by learning to become more like samples in hopes of winning the next
competition. It is this selection and learning process that makes the weights organize
themselves into a map representing similarities.
So with these two components (the sample and weight vectors), how can one order the
weight vectors in such a way that they will represent the similarities of the sample vectors?
This is accomplished by using the very simple algorithm shown here.
Initialize Map
For t from 0 to 1
Randomly select a sample
Get best matching unit
Scale neighbors
Increase t a small amount
End for
Fig. 7. A Sample SOM Algorithm
The first step in constructing a SOM is to initialize the weight vectors. From there you select
a sample vector randomly and search the map of weight vectors to find which weight best
represents that sample. Since each weight vector has a location, it also has neighbouring
weights that are close to it. The weight that is chosen is rewarded by being able to become
more like that randomly selected sample vector. In addition to this reward, the neighbours
of that weight are also rewarded by being able to become more like the chosen sample
vector. From this step we increase t some small amount because the number of neighbours
and how much each weight can learn decreases over time. This whole process is then
repeated a large number of times, usually more than 1000 times.
In the case of colours, the program would first select a color from the array of samples such
as green, then search the weights for the location containing the greenest color. From there,
the colour surrounding that weight are then made more green. Then another color is chosen,
such as red, and the process continues. They processes are:
•
Initializing the Weights
Here are screen shots of the three different ways which decided to initialize the
weight vector map. We should first mention the palette here. In the java program
below there are 6 intensities of red, blue, and green displayed, it really does not
take away from the visual experience. The actual values for the weights are floats,
so they have a bigger range than the six values that are shown in figure 7 below.
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Fig. 8. Weight Values
There are a number of ways to initialize the weight vectors. The first you can see is just give
each weight vector random values for its data. A screen of pixels with random red, blue, and
green values is shown above on the left. Unfortunately calculating SOMs according to
Kohonen (Kohonen, 1997) is very computationally expensive, so there are some variants of
initializing the weights so that samples that you know for a fact are not similar start off far
away. This way you need less iteration to produce a good map and can save yourself some
time.
Here we made two other ways to initialize the weights in addition to the random one. This
one is just putting red, blue, green, and black at all four corners and having them slowly
fade toward the center. This other one is having red, green, and blue equally distant from
one another and from the center.
•
B. Get Best Matching Unit
This is a very simple step, just go through all the weight vectors and calculate the
distance from each weight to the chosen sample vector. The weight with the
shortest distance is the winner. If there are more than one with the same distance,
then the winning weight is chosen randomly among the weights with the shortest
distance. There are a number of different ways for determining what distance
actually means mathematically. The most common method is to use the Euclidean
distance:
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where x[i] is the data value at the ith data member of a sample and n is the number of
dimensions to the sample vectors.
In the case of colour, if we can think of them as 3D points, each component being an axis. If
we have chosen green which is of the value (0,6,0), the color light green (3,6,3) will be closer
to green than red at (6,0,0).
Red
Light
green
=
Sqrt((3-0)^2+(6-6)^2+(3-0)^2)
= Sqrt((6-0)^2+(0-6)^2+(0-0)^2) = 8.49
=
4.24
So light green is now the best matching unit, but this operation of calculating distances and
comparing them is done over the entire map and the wieght with the shortest distance to the
sample vector is the winner and the BMU. The square root is not computed in the java
program for speed optimization for this section.
•
C. Scale Neighbors
1. Determining Neighbors
There are actually two parts to scaling the neighboring weights: determining which
weights are considered as neighbors and how much each weight can become more
like the sample vector. The neighbors of a winning weight can be determined using
a number of different methods. Some use concentric squares, others hexagons, I
opted to use a gaussian function where every point with a value above zero is
considered a neighbor.
As mentioned previously, the amount of neighbors decreases over time. This is
done so samples can first move to an area where they will probably be, then they
jockey for position. This process is similar to coarse adjustment followed by fine
tuning. The function used to decrease the radius of influence does not really matter
as long as it decreases, we just used a linear function.
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Fig. 9. A graph of SOM Neighbour’s determination
Figure 8 above shows a plot of the function used. As the time progresses, the base goes
towards the centre, so there are less neighbours as time progresses. The initial radius is set
really high, some value near the width or height of the map.
2. Learning
The second part to scaling the neighbours is the learning function. The winning
weight is rewarded with becoming more like the sample vector. The neighbours
also become more like the sample vector. An attribute of this learning process is
that the farther away the neighbour is from the winning vector, the less it learns.
The rate at which the amount a weight can learn decreases and can also be set to
whatever you want. I chose to use a gaussian function. This function will return a
value ranging between 0 and 1, where each neighbor is then changed using the
parametric equation. The new color is:
Current color*(1.-t) + sample vector*t
So in the first iteration, the best matching unit will get a t of 1 for its learning
function, so the weight will then come out of this process with the same exact
values as the randomly selected sample.
Just as the amount of neighbors a weight has falls off, the amount a weight can learn also
decreases with time. On the first iteration, the winning weight becomes the sample vector
since t has a full range of from 0 to 1. Then as time progresses, the winning weight becomes
slightly more like the sample where the maximum value of t decreases. The rate at which
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the amount a weight can learn falls of linearly. To depict this visually, in the previous plot,
the amount a weight can learn is equivalent to how high the bump is at their location. As
time progresses, the height of the bump will decrease. Adding this function to the
neighbourhood function will result in the height of the bump going down while the base of
the bump shrinks.
So once a weight is determined the winner, the neighbours of that weight is found and each
of those neighbours in addition to the winning weight change to become more like the
sample vector.
1.4.1 Determining the Quality of SOMs
Below is another example of a SOM generated by the program using 500 iterations in figure
9. At first glance you will notice that similar colour is all grouped together yet again.
However, this is not always the case as you can see that there are some colour who are
surrounded by colour that are nothing like them at all. It may be easy to point this out with
colour since this is something that we are familiar with, but if we were using more abstract
data, how would we know that since two entities are close to each other means that they are
similar and not that they are just there because of bad luck?
Fig. 10. SOM Iteration
There is a very simple method for displaying where similarities lie and where they do not.
In order to compute this we go through all the weights and determine how similar the
neighbors are. This is done by calculating the distance that the weight vectors make between
the each weight and each of its neighbors. With an average of these distances a color is then
assigned to that location. This procedure is located in Screen.java and named public void
update_bw().
If the average distance were high, then the surrounding weights are very different and a
dark color is assigned to the location of the weight. If the average distance is low, a lighter
color is assigned. So in areas of the center of the blobs the colour are the same, so it should
be white since all the neighbors are the same color. In areas between blobs where there are
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similarities it should be not white, but a light grey. Areas where the blobs are physically
close to each other, but are not similar at all there should be black. See Figure 8 below
Fig. 11. A sample allocation of Weight in Colour
As shown above, the ravines of black show where the colour may be physically close to each
other on the map, but are very different from each other when it comes to the actual values
of the weights. Areas where there is a light grey between the blobs represent a true
similarity. In the pictures above, in the bottom right there is black surrounded by colour
which are not very similar to it. When looking at the black and white similarity SOM, it
shows that black is not similar to the other colour because there are lines of black
representing no similarity between those two colour. Also in the top corner there is pink and
nearby is a light green which are not very near each other in reality, but near each other on
the colored SOM. Looking at the black and white SOM, it clearly shows that the two not
very similar by having black in between the two colour.
With these average distances used to make the black and white map, we can actually assign
each SOM a value that determines how good the image represents the similarities of the
samples by simply adding these averages.
1.4.2 Advantages and Disadvantages of SOM
Self organise map has the following advantages:
•
•
Probably the best thing about SOMs that they are very easy to understand. It’s very
simple, if they are close together and there is grey connecting them, then they are
similar. If there is a black ravine between them, then they are different. Unlike
Multidimensional Scaling or N-land, people can quickly pick up on how to use
them in an effective manner.
Another great thing is that they work very well. As I have shown you they classify
data well and then are easily evaluate for their own quality so you can actually
calculated how good a map is and how strong the similarities between objects are.
These are the disadvantages:
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•
•
•
One major problem with SOMs is getting the right data. Unfortunately you need a
value for each dimension of each member of samples in order to generate a map.
Sometimes this simply is not possible and often it is very difficult to acquire all of
this data so this is a limiting feature to the use of SOMs often referred to as missing
data.
Another problem is that every SOM is different and finds different similarities
among the sample vectors. SOMs organize sample data so that in the final product,
the samples are usually surrounded by similar samples, however similar samples
are not always near each other. If you have a lot of shades of purple, not always
will you get one big group with all the purples in that cluster, sometimes the
clusters will get split and there will be two groups of purple. Using colour we
could tell that those two groups in reality are similar and that they just got split,
but with most data, those two clusters will look totally unrelated. So a lot of maps
need to be constructed in order to get one final good map.
The final major problem with SOMs is that they are very computationally
expensive which is a major drawback since as the dimensions of the data increases,
dimension reduction visualization techniques become more important, but
unfortunately then time to compute them also increases. For calculating that black
and white similarity map, the more neighbours you use to calculate the distance
the better similarity map you will get, but the number of distances the algorithm
needs to compute increases exponentially.
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New Advances in Machine Learning
Edited by Yagang Zhang
ISBN 978-953-307-034-6
Hard cover, 366 pages
Publisher InTech
Published online 01, February, 2010
Published in print edition February, 2010
The purpose of this book is to provide an up-to-date and systematical introduction to the principles and
algorithms of machine learning. The definition of learning is broad enough to include most tasks that we
commonly call “learning” tasks, as we use the word in daily life. It is also broad enough to encompass
computers that improve from experience in quite straightforward ways. The book will be of interest to industrial
engineers and scientists as well as academics who wish to pursue machine learning. The book is intended for
both graduate and postgraduate students in fields such as computer science, cybernetics, system sciences,
engineering, statistics, and social sciences, and as a reference for software professionals and practitioners.
The wide scope of the book provides a good introduction to many approaches of machine learning, and it is
also the source of useful bibliographical information.
How to reference
In order to correctly reference this scholarly work, feel free to copy and paste the following:
Taiwo Oladipupo Ayodele (2010). Types of Machine Learning Algorithms, New Advances in Machine Learning,
Yagang Zhang (Ed.), ISBN: 978-953-307-034-6, InTech, Available from:
http://www.intechopen.com/books/new-advances-in-machine-learning/types-of-machine-learning-algorithms
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