Although FIG. 3 illustrates use of cluster-based feature generation to extract features from an unlabeled dataset, in other embodiments, methods such as principal component analysis, described in ???Using Linear Algebra for Intelligent Information Retrieval??? by M. Berry, S. T. Dumais and G. W. O'Brien, SIAM Review 37(4):573-595, incorporated by reference herein in its entirety, or sparse coding,