Beyond Hartigan Consistency: Merge Distortion Metric for Hierarchical Clustering

author: Justin Eldridge, Department of Computer Science and Engineering, Ohio State University
published: Aug. 20, 2015,   recorded: July 2015,   views: 40
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Hierarchical clustering is a popular method for analyzing data, which associates a tree to a dataset. Hartigan consistency has been used extensively as a framework to analyze such clustering algorithms from a statistical point of view. Still, as we show in the paper, a tree which is Hartigan consistent with a given density can look very different than the correct limit tree. Specifically, Hartigan consistency permits two types of undesirable configurations which we term over-segmentation and {\it improper nesting}. Moreover, Hartigan consistency is a limit property and does not directly quantify difference between trees. In this paper we identify two limit properties, separation and minimality, which address both over-segmentation and improper nesting and together imply (but are not implied by) Hartigan consistency. We proceed to introduce a merge distortion distance between hierarchical clusterings and show that convergence in that distance implies both separation and minimality. We also show that uniform separation and minimality imply convergence in the merge distortion distance. Furthermore, we show that our merge distortion distance is stable under perturbations of the density. Finally, we demonstrate applicability of these concepts by proving convergence results for two clustering algorithms. First, we show convergence (and hence separation and minimality) of the recent robust single linkage algorithm of Chaudhuri and Dasgupta (2010). Second, we provide convergence results on manifolds for topological split tree clustering.

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