The Entropy of a cluster reflects how the members of the k distinct subgroups are distributed within each resulting cluster; the global quality measure is computed by averaging the entropy of all clusters: (2) where P (i, j) is the probability of finding an element from the category i in the cluster j, n j is the number of items in cluster j, and n the total number of items in the distribution.
to purity, the entropy decreases as the quality of clustering improves.