Given a text of length nn using an alphabet consisting of cc letters and given the number of appearances fif_i for each letter ii, the Index of coincidence (IC) is defined by the sum:

IC=i=1cfi(fi1)n(n1)\text{IC} = \frac{\sum_{i=1}^cf_i(f_i-1)}{n(n-1)}

The normalized Index of coincidence can be obtained by multiplying the above IC\text{IC} by cc.

This article is a work in progress.

See also