The products count the number of combinations of "n" elements taken two at a time. (Actually this counts each pair twice; the extra factors of 2 occur in both numerator and denominator of the formula and thus cancel out.) Each of the "ni" occurrences of the "i"-th letter matches each of the remaining occurrences of the same letter. There are a total of letter pairs in the entire text, and 1/"c" is the probability of a match for each pair, assuming a uniform random distribution of the characters (the "null model"; see below). Thus, this formula gives the ratio of the total number of coincidences observed to the total number of coincidences that one would expect from the null model.