Hypergeometric distribution

The hypergeometric distribution is a discrete probability distribution that describes the number of successes in a sequence of n draws from a finite population without replacement.

A typical example is the following: There is a shipment of N objects in which D are defective. The hypergeometric distribution describes the probability that in a sample of n distinctive objects drawn from the shipment exactly k objects are defective.

In general, if a random variable X follows the hypergeometric distribution with parameters N, D and n, then the probability of getting exactly k successes is given by

The probability is positive, when k is between max(0, D + n - N) and min(n, D).

The formula can be understood as follows: There are possible samples (without replacement). There are ways to obtain k defective objects and there are ways to fill out the rest of the sample with non-defective objects.

When the population size is large (i.e. N is large) the hypergeometric distribution can be approximated reasonably well with a binomial distribution with parameters N (number of trials) and p = D / N (probability of success in a single trial).