12015-07-02 08:59:33PeterWatsonDescribe FAQ/combinatorics/inst2rep1 here.

An example of 1 instance of 2 replicates is AAABCD ie three repetitions of the same stimulus in the sequence of length 6. There are (6 choose 3) = 20 ways of ordering the repeated stimulus. These are There are 92 stimuli that can be repeated and there would be three distinct simuli not equal to the repeated stimulus in the remaining three positions giving 92 x 20 x 91 x 90 x 89 possible combinations of a single incidence of 2 repeats of a stimulus giving a probability when divided by 92^6 of 0.002.