Find the optimal number of K in clustering

Tahmineh

Hi everyone. 
i want to determine the optimal number of k in clustering. i applied silhouette criteria. but all of my Avg. within centroid distance according to different numbers of k are negative. 
how can i choose the best one between this negative numbers.

Telcontar120

Did you try the x-means operator?  This does not use the silhouette method but instead uses AIC/BIC penalties for adding additional clusters.  It is usually a good operator to check if you don't have a strong a priori preference for your k value.