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Igor Cadez

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NeurIPS Conference 2002 Conference Paper

Learning to Classify Galaxy Shapes Using the EM Algorithm

Sergey Kirshner
Igor Cadez
Padhraic Smyth
Chandrika Kamath

We describe the application of probabilistic model-based learning to the problem of automatically identifying classes of galaxies, based on both morphological and pixel intensity characteristics. The EM algorithm can be used to learn how to spatially orient a set of galaxies so that they are geometrically aligned. We augment this “ordering-model” with a mixture model on objects, and demonstrate how classes of galaxies can be learned in an unsupervised manner using a two-level EM algorithm. The resulting models provide highly accurate classi£cation of galaxies in cross-validation experiments. 1 Introduction and Background The £eld of astronomy is increasingly data-driven as new observing instruments permit the rapid collection of massive archives of sky image data. In this paper we investigate the problem of identifying bent-double radio galaxies in the FIRST (Faint Images of the Radio Sky at Twenty-cm) Survey data set [1]. FIRST produces large numbers of radio images of the deep sky using the Very Large Array at the National Radio Astronomy Observatory. It is scheduled to cover more that 10, 000 square degrees of the northern and southern caps (skies). Of particular scienti£c interest to astronomers is the identi£cation and cataloging of sky objects with a “bent-double” morphology, indicating clusters of galaxies ([8], see Figure 1). Due to the very large number of observed deep-sky radio sources, (on the order of 106 so far) it is infeasible for the astronomers to label all of them manually. The data from the FIRST Survey (http: //sundog. stsci. edu/) is available in both raw image format and in the form of a catalog of features that have been automatically derived from the raw images by an image analysis program [8]. Each entry corresponds to a single detectable “blob” of bright intensity relative to the sky background: these entries are called Figure 1: 4 examples of radio-source galaxy images. The two on the left are labelled as “bent-doubles” and the two on the right are not. The con£gurations on the left have more “bend” and symmetry than the two non-bent-doubles on the right. components. The “blob” of intensities for each component is £tted with an ellipse. The ellipses and intensities for each component are described by a set of estimated features such as sky position of the centers (RA (right ascension) and Dec (declination)), peak density ¤ux and integrated ¤ux, root mean square noise in pixel intensities, lengths of the major and minor axes, and the position angle of the major axis of the ellipse counterclockwise from the north. The goal is to £nd sets of components that are spatially close and that resemble a bent-double. In the results in this paper we focus on candidate sets of components that have been detected by an existing spatial clustering algorithm [3] where each set consists of three components from the catalog (three ellipses). As of the year 2000, the catalog contained over 15, 000 three-component con£gurations and over 600, 000 con£gurations total. The set which we use to build and evaluate our models consists of a total of 128 examples of bent-double galaxies and 22 examples of non-bent-double con£gurations. A con£guration is labelled as a bent-double if two out of three astronomers agree to label it as such. Note that the visual identi£cation process is the bottleneck in the process since it requires signi£cant time and effort from the scientists, and is subjective and error-prone, motivating the creation of automated methods for identifying bent-doubles. Three-component bent-double con£gurations typically consist of a center or “core” com- ponent and two other side components called “lobes”. Previous work on automated classi£- cation of three-component candidate sets has focused on the use of decision-tree classi£ers using a variety of geometric and image intensity features [3]. One of the limitations of the decision-tree approach is its relative in¤exibility in handling uncertainty about the object being classi£ed, e. g. , the identi£cation of which of the three components should be treated as the core of a candidate object. A bigger limitation is the £xed size of the feature vec- tor. A primary motivation for the development of a probabilistic approach is to provide a framework that can handle uncertainties in a ¤exible coherent manner. 2 Learning to Match Orderings using the EM Algorithm We denote a three-component con£guration by C = (c 1; c2; c3), where the ci’s are the components (or “blobs”) described in the previous section. Each component cx is repre- sented as a feature vector, where the speci£c features will be de£ned later. Our approach focuses on building a probabilistic model for bent-doubles: p (C) = p (c1; c2; c3), the like- lihood of the observed ci under a bent-double model where we implicitly condition (for now) on the class “bent-double. ” By looking at examples of bent-double galaxies and by talking to the scientists study- ing them, we have been able to establish a number of potentially useful characteristics of the components, the primary one being geometric symmetry. In bent-doubles, two of the components will look close to being mirror images of one another with respect to a line through the third component. We will call mirror-image components lobe compo-

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NeurIPS Conference 2001 Conference Paper

Bayesian Predictive Profiles With Applications to Retail Transaction Data

Igor Cadez
Padhraic Smyth

Massive transaction data sets are recorded in a routine manner in telecommunications, retail commerce, and Web site management. In this paper we address the problem of inferring predictive in- dividual proﬂles from such historical transaction data. We de- scribe a generative mixture model for count data and use an an approximate Bayesian estimation framework that eﬁectively com- bines an individual’s speciﬂc history with more general population patterns. We use a large real-world retail transaction data set to illustrate how these proﬂles consistently outperform non-mixture and non-Bayesian techniques in predicting customer behavior in out-of-sample data.

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NeurIPS Conference 2001 Conference Paper

Model Based Population Tracking and Automatic Detection of Distribution Changes

Igor Cadez
P. S. Bradley

Probabilistic mixture models are used for a broad range of data anal- ysis tasks such as clustering, classiﬁcation, predictive modeling, etc. Due to their inherent probabilistic nature, mixture models can easily be combined with other probabilistic or non-probabilistic techniques thus forming more complex data analysis systems. In the case of online data (where there is a stream of data available) models can be constantly up- dated to reﬂect the most current distribution of the incoming data. How- ever, in many business applications the models themselves represent a parsimonious summary of the data and therefore it is not desirable to change models frequently, much less with every new data point. In such a framework it becomes crucial to track the applicability of the mixture model and detect the point in time when the model fails to adequately represent the data. In this paper we formulate the problem of change detection and propose a principled solution. Empirical results over both synthetic and real-life data sets are presented. 1 Introduction and Notation Consider a data set D = fx1; x2; :: :; xng consisting of n independent, identically dis- tributed (iid) data points. In context of this paper the data points could be vectors, se- quences, etc. Further, consider a probabilistic mixture model that maps each data set to a real number, the probability of observing the data set:

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NeurIPS Conference 2000 Conference Paper

Model Complexity, Goodness of Fit and Diminishing Returns

Igor Cadez
Padhraic Smyth

We investigate a general characteristic of the trade-off in learning problems between goodness-of-fit and model complexity. Specifi(cid: 173) cally we characterize a general class of learning problems where the goodness-of-fit function can be shown to be convex within first(cid: 173) order as a function of model complexity. This general property of "diminishing returns" is illustrated on a number of real data sets and learning problems, including finite mixture modeling and multivariate linear regression. Introduction, Motivation, and Related Work 1 Assume we have a data set D = {Xl, X2, .. ., x n }, where the X i could be vectors, sequences, etc. We consider modeling the data set D using models indexed by a complexity index k, 1: :: ; k: :: ; kmax • For example, the models could be finite mixture probability density functions (PDFs) for vector Xi'S where model complexity is indexed by the number of components k in the mixture. Alternatively, the modeling task could be to fit a conditional regression model y = g(Zk) + e, where now y is one of the variables in the vector X and Z is some subset of size k of the remaining components in the X vector. Such learning tasks can typically be characterized by the existence of a model and a loss function. A fitted model of complexity k is a function of the data points D and depends on a specific set of fitted parameters B. The loss function (goodness(cid: 173) of-fit) is a functional of the model and maps each specific model to a scalar used to evaluate the model, e. g. , likelihood for density estimation or sum-of-squares for regression. Figure 1 illustrates a typical empirical curve for loss function versus complexity, for mixtures of Markov models fitted to a large data set of 900, 000 sequences. The complexity k is the number of Markov models being used in the mixture (see Cadez et al. (2000) for further details on the model and the data set). The empirical curve has a distinctly concave appearance, with large relative gains in fit for low complexity models and much more modest relative gains for high complexity models. A natural question is whether this concavity characteristic can be viewed as a general phenomenon in learning and under what assumptions on model classes and Nwnber of M Ixture Cmnponen1S 11] Figure 1: Log-likelihood scores for a Markov mixtures data set. loss functions the concavity can be shown to hold. The goal of this paper is to illustrate that in fact it is a natural characteristic for a broad range of problems in mixture modeling and linear regression. We note of course that for generalization that using goodness-of-fit alone will lead to the selection of the most complex model under consideration and will not in general select the model which generalizes best to new data. Nonetheless our pri(cid: 173) mary focus of interest in this paper is how goodness-of-fit loss functions (such as likelihood and squared error, defined on the training data D) behave in general as a function of model complexity k. Our concavity results have a number of interesting implications. For example, for model selection methods which add a penalty term to the goodness-of-fit (e. g. , BIC), the resulting score function as a function of model complexity will be unimodal as a function of complexity k within first order. Li and Barron (1999) have shown that for finite mixture models the expected value of the log-likelihood for any k is bounded below by a function of the form -C /k where C is a constant which is independent of k. The results presented here are complementary in the sense that we show that the actual maximizing log-likelihood itself is concave to first-order as a function of k. Furthermore, we obtain a more general principle of "diminishing returns, " including both finite mixtures and subset selection in regression.

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