The Area Under the the Receiver Operating Characteristics (ROC) Curve,
referred to as AUC, is a well-known performance measure in the supervised
learning domain. Due to its compelling features, it has been employed in a
number of studies to evaluate and compare the performance of different
classifiers. In this work, we explore AUC as a performance measure in the
unsupervised learning domain, more specifically, in the context of cluster
analysis. In particular, we elaborate on the use of AUC as an internal/relative
measure of clustering quality, which we refer to as Area Under the Curve for
Clustering (AUCC). We show that the AUCC of a given candidate clustering
solution has an expected value under a null model of random clustering
solutions, regardless of the size of the dataset and, more importantly,
regardless of the number or the (im)balance of clusters under evaluation. In
addition, we elaborate on the fact that, in the context of internal/relative
clustering validation as we consider, AUCC is actually a linear transformation
of the Gamma criterion from Baker and Hubert (1975), for which we also formally
derive a theoretical expected value for chance clusterings. We also discuss the
computational complexity of these criteria and show that, while an ordinary
implementation of Gamma can be computationally prohibitive and impractical for
most real applications of cluster analysis, its equivalence with AUCC actually
unveils a much more efficient algorithmic procedure. Our theoretical findings
are supported by experimental results. These results show that, in addition to
an effective and robust quantitative evaluation provided by AUCC, visual
inspection of the ROC curves themselves can be useful to further assess a
candidate clustering solution from a broader, qualitative perspective as well.Comment: 37 pages, 5 figures, submitted for publicatio