lecture 7 - Cluster Analysis Prof Thomas B Fomby Department...

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1 Cluster Analysis Prof. Thomas B. Fomby Department of Economics Southern Methodist University Dallas, TX 75275 April 2008 April 2010 Cluster Analysis , sometimes called data segmentation or customer segmentation , is an unsupervised learning method . As you will recall a method is an unsupervised learning method if it doesn’t involve prediction or classification. The major purpose of Cluster Analysis is to group together collections of objects (e.g. customers) into “clusters” so that the objects in the clusters are “similar.” One reason a company might want to organize its customers into groups is to come to better understand the nature of its customers. Given the delineation of its customers into distinct groups, the company could advertise differently to its distinct groups, send different catalogues to its distinct groups, and the like. In terms of building prediction and classification models, cluster analysis can help the analyst identify groups of input variables that in turn can lead to different models for each group. This is, of course, assuming that the output relationships vis-à-vis the input variables across the groups are not the same. But then one can always test the “poolability” of the models by either conventional hypothesis tests, when considering econometric models, or accuracy measures across validation and test data partitions when considering machine learning models. As one will come to understand after working on several clustering projects, clustering is an “Art Form.” It mu st be practiced with care. The more experience you have in doing cluster analysis, the better you become as a practitioner. Before beginning cluster analysis it is often recommended that the data be normalized first. Cluster analysis based on variables with very different scales of measurement can lead to clusters that are not very robust to adding or deleting variables or observations. In this discussion, we will be focusing on clustering only continuous input variables . The clustering of mixed data, some continuous and some categorical, is not considered here as it is beyond the scope of this discussion. Now let us begin. There are two basic approaches to clustering: a) Hierarchical Clustering (Agglomerative Clustering discussed here) b) Non-hierarchical clustering (K-means)
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2 Hierarchical Clustering With respect to hierarchical clustering, the final clusters chosen are built in a series of steps. If we start with N objects, each being in its own separate cluster, and then combine one of the clusters with another cluster resulting in N 1 clusters and continue to combine clusters into fewer and few clusters with more and more objects in each cluster, we are engaging in Agglomerative clustering . In contrast, if we start with all of the objects being in a single cluster and then remove one of the objects to form a second cluster and then continue to build more and more clusters with fewer and few objects in each cluster until each object is in its own cluster, we are engaging in Divisive
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