Thanks - sklearn works great! I got exactly what I expected each time I ran it! James import sys import numpy from sklearn.cluster import * print sys.version vals = numpy.array([[0.0],[0.1],[0.5],[0.6],[1.0],[1.1]]) print vals k_means_ex = KMeans(k=3) x = k_means_ex.fit_predict(vals) print x print k_means_ex.cluster_centers_ print k_means_ex.score(vals) 2.7.3 (default, Apr 10 2012, 23:31:26) [MSC v.1500 32 bit (Intel)] [[ 0. ] [ 0.1] [ 0.5] [ 0.6] [ 1. ] [ 1.1]] [1 1 0 0 2 2] [[ 0.55] [ 0.05] [ 1.05]] -0.015