http://en.wikipedia.org/wiki/Determining_the_number_of_clusters_in_a_data_set http://en.wikipedia.org/wiki/Determining_the_number_of_clust... It's "tough luck"

http://en.wikipedia.org/wiki/Determining_the_number_of_clusters_in_a_data_set http://en.wikipedia.org/wiki/Determining_the_number_of_clust... It's "tough luck" because the algorithms determining the number of clusters aren't magical. As with all machine learning, the results reflect the assumptions built into the algorithm.