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\title{Promises and Challenges in Automatic Pattern Recognition}
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\author{Tin Kam Ho}
\affil{Bell Laboratories, Lucent Technologies}
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\contact{Tin Kam Ho}
\email{tkh@research.bell-labs.com}
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\keywords{data: visualization, interactive: graphics, pattern recognition,
cluster, classification, data: mining}
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\begin{abstract}
Pattern recognition is to identify and model
regularities in empirical data by algorithmic processes.
Successful application of the established methods
requires good understanding of their behavior and
how well they match a particular context.
Difficulties can arise from either the intrinsic complexity
of a problem or a mismatch of methods to problems.
We describe our efforts in characterizing the intrinsic complexity
of a classification problem and its relationship to classifier
performance. We discuss how Mirage, an exploratory data analysis
tool, is designed to help integrate domain expertise into the process
of solution development.
\end{abstract}
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\section{Introduction}
Many large-scale sky surveys in progress or in planning are expected to
generate data at a rate far beyond reach by traditional manual
analysis. Inevitably some form of automatic analysis must be employed
at a certain stage of the data processing pipeline. Ideally,
automatic analysis should go beyond routine data reduction operations,
and play an active role in assisting and accelerating the process of
knowledge discovery.
Here astronomy shares similar concerns with many other areas of
study, like weather forecasting, earth observation, robotic perception,
medical diagnosis, security monitoring, web-based information
retrieval, and financial engineering. There are similar needs for
processing numbers, time series, images, sound, and spectral
observations to extract useful information and detect new events. One
may even argue that, the ability to recognize patterns from
observations is at the heart of human intelligence. Besides these
explicit applications, many of our daily activities like physical
navigation, social interactions, and decision making, all depend
critically on this ability.
Algorithmic methods for discovering regularities and irregularities in
data sets have been under active research over the entire history of
development in computing machinery. In some application domains there
have been good progresses resulting in successful algorithms in
routine use, such as in postal address reading machines for high-speed,
large-volume mail sorting. But obviously we are still far from
solving all potential application problems. In this article we review
what have been accomplished in the methodology, and what challenges are ahead.
\section{Essential Tasks and Methods in Automatic Pattern Recognition}
The process of automatic pattern recognition involves several key tasks:
choice of a good feature representation,
design of procedures for feature extraction,
selection of relevant features for classification,
and from there it can go into supervised or unsupervised learning
(Jain et al. 2000).
In supervised learning (discrimination), we use a labeled training set
to tune a chosen classifier so that it can assign an unseen sample to
one of the several pre-defined classes exemplified in the training set.
This involves choosing a classifier (an algorithm among several known
families), training the classifier (developing the necessary data
structures or tuning the critical parameters), and evaluating the
classifier to set expectation on its accuracy before the actual
application. Success of the process is measured by the accuracy
in class assignment on a new data set.
In unsupervised learning (clustering), there are no pre-defined
classes. We need to choose and tune an algorithm to find clusters that
are subsets of data sharing some common properties.
This involves choosing and tuning a clustering algorithm, and
evaluating the results within the application context. Good results
are those satisfying a specific validation criterion associated with
the algorithm, or those with useful interpretations in the context of
the domain knowledge.
\subsection*{Feature Extraction and Representation}
Feature extraction is the process of obtaining measurements of the
objects. The measurements can come directly from sensor output,
or can be reduced and refined by domain-specific algorithms
or generic tools like Gabor analysis, Fourier analysis, wavelet analysis,
or principle component analysis. Features need to be informative of
the task at hand. This is where domain expertise can do most
help: which shape features are the most critical descriptors of
galaxy morphology? should a set of light curve be normalized by
maximum intensity or by duration per cycle? is an object's
position important for identification of its type?
which coordinate system is the most appropriate for this task?
A good set of features have strong impact on the success of the
subsequent recognition process.
In representing the features there are several choices:
(1) fixed length numerical vectors; (2) symbol strings; (3) graphs and
other data structures. There is a depository of recognition methods
matching each feature representation. In addition, there are
rule-based classifier systems that can handle different kinds of
feature representations. The rules can be hand-designed, deduced by
formal logic, or selected by genetic algorithms.
\subsection*{Classifiers and Clustering Algorithms}
Symbol strings are typically processed by syntactic pattern recognition
methods. These methods represent each class by a grammar consisting
of a set of allowed symbols and production rules. The classifier is a
parser that tries to reduce the input string to the most likely
generating symbol according to the production rules.
Syntactic methods are best suited to problems with clear primitives
and stable intermediate structures, with well defined and known
alternatives. Domain knowledge can be explicitly encoded
into the grammar (Trahanias \& Skordalakis 1990).
However, if good knowledge about the problem's structure is not
available, automatically inferring the grammar from a sample set
is a very difficult task.
% a recent survey?
Features represented by graphs are processed by structural methods
like graph matching for isomorphisms, elastic matching, and
subtree matching. Both the data structure and the matching procedure
can encode knowledge about the problem. Careful heuristic designs are
needed in most applications. Difficulties include how to build noise
tolerance into the matching algorithms, and how to deal with the
combinatorial complexity.
Fixed length numerical vectors are the most commonly used feature
representations. Feature vectors are processed by statistical
pattern recognition methods. Table \ref{O3-1:methods} lists
several major families of classification methods in this category.
Statistical pattern recognition methods do
not rely on explicit encoding of domain knowledge. The classifier
training procedures include mechanisms for inferring the distribution
of each class in the
feature space. This makes them widely applicable, and they are by far
the richest and the most successful category of methods in practice.
However, even with a wide range of choices in classifier algorithms,
there is no guarantee of success for any particular application.
Often the difficulty is in finding the right method best suited for
the problem at hand (Figure~\ref{O3-1:boundary}).
\begin{table}[p]
\center
\footnotesize
\begin{tabular}{|l|l|l|} \hline
method & principle & variants \\ \hline
\parbox{0.6in}{
Bayesian classifiers}
& \parbox{1.5in}{
estimate the class-conditional distribution using the training samples;
assign an unknown sample to the class with maximum {\em a posteriori}
probability
} & \parbox{2.75in}{
(1) parametric procedures: estimates of probability distributions
are based on an assumed functional form (e.g. Gaussian);
(2) nonparametric procedures: no functional form of the
distributions is assumed; the empirical distributions are
described using generic estimators (e.g. based on kernels) or low-order
approximations based on weak assumptions such as
continuity and smoothness }\\ \hline
\parbox{0.6in}{
linear classifiers}
& \parbox{1.5in}{
infer a hyperplane best separating two classes (or each class from the rest);
assign an unknown sample to the side of the hyperplane
believed to contain its class
} & \parbox{2.75in}{
(1) many variations in the procedure of inferring the hyperplane:
Fisher's discriminant analysis, equi-distance divider between class
means (nearest mean classifier), minimizing sum of error distances,
maximizing margins, or by iterative weight adjustments minimizing errors.
(2) by applying nonlinear transformations on the original features,
the problem can be embedded in a new space where a linear classifier
can describe a nonlinear boundary in the original space
(as in support vector machines)} \\ \hline
\parbox{0.6in}{
nearest neighbor classifiers}
& \parbox{1.5in}{
assign an unknown sample to the class represented by training samples
in its vicinity
} & \parbox{2.75in}{
(1) variations by the distance metric;
(2) K-nearest neighbors: take votes from the classes of several nearest
neighbors;
(3) reduced, condensed classifiers where only a subset of the training
samples are used, mostly for efficiency concern} \\ \hline
\parbox{0.6in}{
decision trees \& forests }
& \parbox{1.5in}{
construct a tree representing a hierarchical partitioning of the feature
space into leaves where a single class dominates;
propagate an unknown sample down the tree to a particular leave node and
assign it to the class with maximum probability at the leave node
} & \parbox{2.75in}{
(1) variations by the type of split at each internal node;
(2) variations by the algorithm for tree construction: how features and
their weights are chosen at each split, when to stop growing the
tree, whether the tree is pruned back for better generalization power;
(3) decision forests: voting by an ensemble of decision trees,
each tree differing in training samples or subsets of features used}
\\ \hline
\parbox{0.6in}{
neural networks}
& \parbox{1.5in}{
design and weigh connections between groups of source, target, and
intermediate nodes representing hidden structures;
propagate an unknown sample through the network until it reaches the
target group, threshold or take maximum of accumulated scores to
decide on its class
} & \parbox{2.75in}{
(1) architectural variations:
multi-layer perceptrons, radial basis networks, Hopfield nets,
learning vector quantization;
(2) differences in several aspects of the training procedure:
target function for optimization, algorithm for weight adjustment,
validation procedure for termination of training, and network size control}
\\ \hline
\parbox{0.6in}{
ensemble methods}
& \parbox{1.5in}{
train a committee of classifiers together with a decision combination
rule;
feed an unknown sample to each component classifier and combine their
decisions on its class
} & \parbox{2.75in} {
(1) variations in the ensemble size and the mixture of classifier
types;
(2) differences in the procedures for constructing and optimizing the ensemble
(3) variations in the decision combination rules } \\ \hline
\end{tabular}
\caption{Common methods for supervised classification.}
\label{O3-1:methods}
\end{table}
\begin{figure}[hbt]
\epsscale{.22}
\centering
\begin{tabular}{cccc}
\plotone{O3-1_f1a.eps} &
\plotone{O3-1_f1b.eps} &
\plotone{O3-1_f1c.eps} &
\plotone{O3-1_f1d.eps}
\end{tabular}
\begin{tabular}{cccc}
\hspace{-0.1in}
(a) &
\hspace{0.85in}
(b) &
\hspace{0.8in}
(c) &
\hspace{0.8in}
(d)
\end{tabular}
\caption{Class boundary according to different classifiers:
(a) an example two-class, two-dimensional problem; class
boundary according to (b) XCS classifier (a genetic algorithm),
(c) nearest-neighbor classifier, and (d) a linear classifier.}
\label{O3-1:boundary}
\end{figure}
For unsupervised learning there are also a large collection of
algorithms, including parametric methods like mixture estimation
(most commonly Gaussian mixtures), and nonparametric methods like
the k-means algorithm, mode seeking procedures (finding density peaks or
valleys), self-organizing maps (a neural network), graph based methods
(minimal spanning tree), and hierarchical methods.
Many of these methods have found good uses in some problems.
However, like in supervised learning,
matching the methods to problems still presents major difficulty,
since each method has certain implicit assumptions about the data
distributions. Coupled with the lack of an absolute notion of
correctness, success in unsupervised learning is often difficult to
determine.
The uncertainty in matches between methods and problems reduces the
application of learning algorithms to a lengthy trial-and-error
process, which is far from desirable for processing
large surveys where many potential questions can be formulated on the
combinatorial interactions of many parameters. The uncertainty is
rooted in a lack of understanding on how data distributions
interact with classifier geometry and the sampling processes.
We believe that the key to improve upon the
current level of automation in pattern learning is a better
understanding of data set complexity in high-dimensional spaces,
especially, the geometry of data distributions and its
detailed relationship to classifier behavior.
In the next section we describe some of our recent studies along these
lines.
\section{Characterization of Data Complexity by Geometrical Measures}
Given an arbitrary discrimination problem, how do we know whether
it is intrinsically solvable by the automatic methods? Given an arbitrary
clustering problem, how do we know whether there are indeed distinct
clusters and which algorithm has the best chance of finding them?
If we do not expect distinct classes, how do we recognize other
types of regularities, such as a trend in evolution, a specific
trajectory in a state space, a compact, low-dimensional distribution in a
high-dimensional feature space? How can we find out about
any irregularities that may indicate new facts?
How do we do all these if the patterns may be buried among irrelevant
data and measurements?
To answer many of these questions it requires a detailed understanding
of how the data are distributed in the feature space. A pattern is
formed if there are data points sharing certain similar attributes. In
geometrical terms it means that these points are close to each other
along some spatial dimensions. Classification is possible when the
points considered to be in the same class are located in compact (dense) groups
within geometrical regions with simple shapes, so that the
gaps left between can accommodate a simple decision boundary.
Such a geometrical perspective is especially helpful if we consider that
most classifiers can also be described by simple geometrical primitives,
such as hyperplanes in linear classifiers, Voronoi regions in
nearest neighbor classifiers, or piecewise linear surfaces in decision
trees.
In a recent study (Ho \& Basu 2002) with several measures of the
geometrical complexity of data sets (Figure~\ref{O3-1:geometry}),
we find that a collection of classification problems arising from
real-world applications can span
a large range in the values of these measures. These problems present
different degrees of difficulty to different kinds of classifiers.
An analysis of the complexity of the problems for which different
classifiers perform the best reveals that classifiers have distinct
domains of competence in the space of the complexity measures
(Ho 2002, PAA). We expect that more complete and systematic studies of this
kind will enable automatic matching of problems to classifiers with
good confidence. The complexity measures can also be used to guide
the formulation of a classification problem, including definition of
the classes, selection of most discriminatory features, and
construction of useful feature transformations.
The complexity of a class boundary may interact with other factors
that also affect a problem's difficulty. These include the intrinsic
ambiguity of classes, due to poor class definitions or poor feature
choices (Figure~\ref{O3-1:ambiguity}), and sample sparsity. Real
applications often contain a mixture of these difficulties
(Figure~\ref{O3-1:sparsity}).
\begin{figure}[hbt]
\epsscale{0.25}
\centering
\begin{tabular}{ccc}
\plotone{O3-1_f2a.eps} &
\hspace{0.25in}
\plotone{O3-1_f2b.eps} &
\hspace{0.25in}
\plotone{O3-1_f2c.eps}
\end{tabular}
\begin{tabular}{ccc}
%\hspace{0.5in}
(a) &
\hspace{1.25in}
(b) &
\hspace{1.25in}
(c)
\end{tabular}
\caption{Different ways for describing the complexity of a classification
boundary:
(a) measure of separability of the convex hulls enclosing two classes
by a particular linear surface;
(b) a count of class-crossing edges in a minimum spanning tree connecting
all the points; (c) a count of maximal balls needed to cover all
points in each class. }
\label{O3-1:geometry}
\end{figure}
\begin{figure}[ht]
\centering
\begin{tabular}{cc}
\epsscale{0.125}
\plotone{O3-1_f3a.eps} &
\hspace{0.25in}
\epsscale{0.25}
\plotone{O3-1_f3b.eps}
\end{tabular}
\caption{Class ambiguity due to different reasons. (Left) instrinsic
shape ambiguity: lower-case letter ``el'' and numeral ``one'' appear
in the same shape in many fonts; the identity can only be determined
from context.
(Right) non-informative features: there may be sufficient features to
classify the shells by shape, but not by the time they were collected
or by which hand they were collected. More informative features are
needed.}
\label{O3-1:ambiguity}
\end{figure}
\begin{figure}[ht]
\centering
\begin{tabular}{cc}
\epsscale{0.25}
\plotone{O3-1_f4a.eps} &
\hspace{0.25in}
\epsscale{0.25}
\plotone{O3-1_f4b.eps}
\vspace{-0.25in}
\end{tabular}
\caption{Complex class geometry and sparse sample cause ill-defined
boundary. More training samples are needed.}
\label{O3-1:sparsity}
\end{figure}
When there is a prior expectation that the data may form
several coherent groups, clustering methods can help discover such
structures. However, many clustering algorithms have strong biases on
the types of structures to look for, to the extent that they may
coerce the data into undesirable groupings that do not necessarily have
clear physical interpretation. One way to guard against such
algorithmic artifacts is to first establish the existence of any
clustering tendency by statistical testing of a uniformity hypothesis.
Similar tests are needed for evidences of more sophisticated structures.
Sometimes the main concern in data analysis is not necessarily to
divide the data into disjoint classes, but rather, to explore
if there exist any outliers, or whether variations in
the data can be described by a trend with a small number of parameters.
Methods for estimating the intrinsic dimensionality (Verveer \& Duin
1995) attempt to determine how many fundamental parameters there are
that determine the variations in the data. This can assist the choice
of a low-dimensional smooth surface for modeling the data, and
outliers can then be detected by assessing their match to the model.
Many of these algorithms have difficulty scaling up to high
dimensionality, and they are still under active research.
A challenge ahead is to establish better connections among the various
methods for characterizing data geometry, and connections between
data geometry and classifier/model geometry. This can help reveal the
limitations of the current methods and suggest how they can be overcome.
The goal for research in this area is to develop a rich language for
geometrical reasoning about point sets in high-dimensional spaces.
We expect this to draw input from progresses in differential geometry
and its variants incorporating stochastic and discrete processes.
\section{Exploratory Visualization of Structures in Data Sets}
Given many open challenges in fully automating the pattern discovery
process, a strategy with well-proven value is to encourage continuous
interaction between domain experts and developers of pattern
recognition algorithms.
Domain expertise can bring insights into many
stages of solution development. An example is, in classifying
galaxies by morphology, how should one apply suitable normalization to
the shape measurements to account for variations in the orientation of
the rotational axis? Also, for validating discrimination between stars
and galaxies, what is the expected luminosity function of each
category? In examining patterns in a set of light
curves, is the maximum intensity a critical feature to consider?
Or should the time series be normalized by the length of each cycle?
If a spectral shape can be described by several key parameters,
can we see their correlations with other known effects, such as the
temperature and kinematic estimates?
A good way to enable the scientists to participate in the process
of algorithm development is through the use of an interactive data
visualization tool. Ideally, the tool can display data and
the results of automatic analysis simultaneously in many different
views to support explorations of a wide range of possibilities. This
can bring input into many stages of solution development, such as:
\begin{itemize}
\item sanitary checking in data preparation: verify correctness of
data reduction steps, clean up undesirable artifacts, and select
most relevant sets of samples;
\item initial exploration: spot explicit patterns, select
potentially useful features, try different normalization schemes,
suggest choices of classifiers, clustering algorithms, or trend models;
\item tentative modeling: examine assigned classes, detected
structures, or identified outliers, compare results of alternative
methods, fine-tune class definitions and algorithm parameters;
\item interpretation: validate classification, interpret detected
structures and trends, correlate results with known facts.
\end{itemize}
\subsection*{Mirage}
The {\bf Mirage} tool (Ho 2003, ADASS) is designed to address these
concerns and needs. Mirage is a software experiment on the displays
and operations that are most suited to enable pattern discovery from
data in multiple types such as numerical vectors, time series, images,
and spectra. Many queries about object properties and the
relationship between different objects can be translated into
geometrical queries on proximity structures in different subspaces
(Figure~\ref{O3-1:walk}), which can be investigated graphically in Mirage.
The features we experiment with include the followings.
\begin{figure}[bt]
\centering
\begin{tabular}{c}
\epsscale{0.5}
\plotone{O3-1_f5.eps}
\end{tabular}
\caption{If data projected to two spaces form different cluster
structures, questions arise on how the structures correlate with each other.
Say, will a walk following a principal curve in one space correspond
to a uni-directional walk in another space? Suppose the structure on the
left represents groups of objects by color, and the one on the right
represents groups by size. We may ask, do objects with the same
color always have the same size? When the object sizes increase
monotonically, how do their colors change?}
\label{O3-1:walk}
\end{figure}
\begin{figure}[bt]
\epsscale{0.475}
\centering
\begin{tabular}{cc}
\plotone{O3-1_f6a.eps} &
\plotone{O3-1_f6b.eps}
\end{tabular}
\caption{Screenshots from Mirage: multiple views of the same
data set can be opened in an arbitrary layout. Selection from
one display can be tracked in other displays.
(Left) a table view, two scatter plots, and a time series display in
parallel coordinates; (Right) an array of histograms each on a different
attribute.}
\label{O3-1:mirage}
\end{figure}
\subsubsection*{Simultaneous, multiple views in a flexible layout.}
For the primary goal of visualizing data geometry, several types
of displays are provided, including basic tools in statistical
graphics like histograms, scatter plots, and parallel coordinate
plots. The displays are organized as a set of pages; each page can
be arbitrarily tiled and any display can go into any tile by
drag-and-drop (Figure~\ref{O3-1:mirage}).
Displays for special data types, such as images in FITS format, can be
plugged in via a standard interface (Carliles et al. 2004).
\subsubsection*{Core exploration commands and display-specific
manipulations.}
A set of core commands are implemented by each display module. These
are for broadcasting the local selection of a set of data points,
highlighting or coloring a broadcast selection, deleting the
selection, canceling the highlights or colors, and for switching
between monochrome and coloring display modes. In addition, each
module can provide local operations suitable for the specific data
type, such as manipulation of image color maps, changing resolution of
histograms, and zooming in and out of a specific display focus.
The selection broadcasting mechanism is very effective for
tracking correlations. Items of interest found from one view
of the data set can be easily traced in other views.
Coordinated sequences of selection and broadcasting can be used
to track the effects of systematic changes in data attributes. An
example is a step-by-step broadcasting of a standard traversal of a
cluster tree, or a walk on each bin of a histogram. When other
displays highlight the corresponding data items, one can easily find out
if the items are also similar in terms of other attributes.
\subsubsection*{Facilities for examining tentative modeling.}
A tentative classification can be represented as one column in
the data matrix, and displayed as one attribute. Selection in that
display can be broadcasted to other displays where the effects of the
classification can be examined. This way one can easily compare
alternative classifications to refine the problem formulation, select
features for the classifier, and polish the classifier design.
Moreover, data can be imported on the fly by adding columns and rows
to the data matrix. Columns added can show newly observed or computed
attributes, including classification decisions
or predictions from external algorithms. Rows added can be
additional samples from the same source, which can be used to
check the correctness of a computed model. Continuous adding and
removing rows can turn the software into a monitoring tool.
\subsubsection*{Extensible software design.}
The software is designed to be extensible in different ways.
Specialized displays can be added as plug-in's.
Exploratory actions are organized around a small command interpreter,
which can be activated via an application interface.
In addition, the software has a slot for plugging in an ``Action''
panel, which can be used to encapsulate data updating and analysis operations
useful for the current task. The Mirage window can also be embedded
into and invoked by wrapper programs which can implement more sophisticated
data access methods (Carliles et al. 2004).
\subsubsection*{Scripting for repeatable, cooperative investigation.}
Data analysis is seldom an individual effort. To enable easy
repetition and sharing of exploration commands and results in a
research team, Mirage provides text-driven commands and
scripting facilities. Scripts can be passed around for replay.
They can also be systematically constructed by simple programs
to make animations.
\vspace{0.1in}
Early applications of Mirage confirmed the effectiveness of many of
these designs, and suggested improvements in several ways.
Programmable display layouts, software state saving and restoration,
and automated explorations are among the most desired. We are also
investigating better ways to cooperate with external analysis code
while maintaining a simple and modular core architecture.
\section{Conclusions}
Research in automatic pattern recognition has resulted in the
identification of several key tasks and the development of a large
collection of methods. Yet we are still missing systematic
procedures for matching methods to problems, largely because of a lack
of effective ways for characterizing data complexity. Early investigations
reveal the multi-facet nature of data complexity and its relationship
to classifier performance. A better understanding of the data
geometry in high-dimensional spaces is needed.
Convenient visualization tools can provide better insight into
the data geometry, and to provide domain experts easy access to the
analysis process. They can closely monitor each stage of solution
development, and offer frequent feedback in key steps like feature
selection, evaluation of tentative classifications, and validation of
final results. The Mirage software is designed to enable such
exchanges. It is constructed as an extensible platform for
supporting knowledge discovery in various degrees of automation.
We are in continuous experimentation on ways to improve it towards a
rich tool for interactive pattern recognition.
\acknowledgments
I thank E.B. Mansilla for the classifier examples in Figure 1,
and many early users of Mirage for their encouragement and suggestions.
\begin{references}
\reference Carliles, S., Ho, T.K., O'Mullane, W.\
2004, \adassxiii \paperref{P3-12}.
\reference Ho, T.K.\ 2002,
Pattern Analysis \& Applications,
{\bf 5}, 102-112.
\reference Ho, T.K. 2003,\ \adassxii \adassref{xii:O6-2}{339}.
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