K-means clustering

K-means is a non-hierarchical classification that optimizes the allocation of data to a specified number of classes
It first “seeds” the data space with proposed centroids, assigning each record to the nearest seed
Once all records are assigned, the centroid for each group is calculated and then used as the seeds for the next round, iteratively until stability
Two K-means runs on the same data will not necessarily produce the same assignments, but divergences are small
I've run these in SPSS, R, and PAST
SPSS produces absurd results
PAleontological STatistics (PAST) is easy, free, and reliable:
http://folk.uio.no/ohammer/past
Hammer, Ø.; Harper, D.A.T.; and Ryan, P. 2001. Paleontological statistics software package for education and data analysis. Palaeontologia Electronica 4, 1: 9 pp.