Luca I.G. Toldo wrote: I agree fully with all the comments previously reported on the challenging topic you raised. You might want to have a look at the following: - Rand index (implemented in "R" http://www.r-project.org/ <http://www.r-project.org/> , in the package e1071 subroutine classAgreement) J.~Cohen. A coefficient of agreement for nominal scales. Educational and Psychological Measurement, 20,37-46, 1960. Lawrence Hubert and Phipps Arabie. Comparing partitions. Journal of Classification, 2, 193-218, 1985. - Linear measure of cluster quality Mather L.A. JOURNAL OF THE AMERICAN SOCIETY FOR INFORMATION SCIENCE. 51(7):602-613,2000 - amendment of the above paper Toldo L, JOURNAL OF THE AMERICAN SOCIETY FOR INFORMATION SCIENCE AND TECHNOLOGY,52(7):602, 2001 -Java implementation of the linear measure of cluster quality Toldo L, Cluster Validity Abstract # 83 Third International Meeting on Microarray Data Standards, Annotations, Ontologies and Databases March 29-31, 2001, Stanford University simply email me if you want a copy of the Java implementation of the LAMCQ algorithm. Regards Dr. Toldo Oops... It appears that my mail filtering system decided that postings from that list were SPAM and I missed the whole discussion :( Rand index (implemented by classAgreement() in R) is exactly what I need. Unfortunately, because of the stupid mail filter I discovered it after I already implemented one myself...