hypergeometric distribution
hypergeometric distribution, in statistics, distribution function in which selections are made from two groups without replacing members of the groups. The hypergeometric distribution differs from the binomial distribution in the lack of replacements. Thus, it often is employed in random sampling for statistical quality control. A simple everyday example would be the random selection of members for a team from a population of girls and boys.
In symbols, let the size of the population selected from be N, with k elements of the population belonging to one group (for convenience, called successes) and N − k belonging to the other group (called failures). Further, let the number of samples drawn from the population be n, such that 0 ≤ n ≤ N. Then the probability (P) that the number (X) of elements drawn from the successful group is equal to some number (x) is given by 
using the notation of binomial coefficients, or, using factorial notation, 
The mean of the hypergeometric distribution is nk/N, and the variance (square of the standard deviation) is nk(N − k)(N − n)/N2(N − 1).