Partition index is a measure ofvalidity similar to partition coeGcient, based on using Pj = ci=1 (uij)m as a

measure ofhow well the jth data point has been classi-

2ed. The closer a pixel is to a codebook entry, the closer

Pj

is to one. Ifa pixel becomes an outlier to all cluster centers, the value of Pj approaches 1=cm − 1, which is the minimum value it can have.