What is hyper-geometic probability?

Answer 1

The hyper-geometric distribution is a discrete probability distribution which is similar (in some respects) to the binomial distribution. Suppose you have a population of N which contains R successes. The Binomial describes the probability of r successes in n draws out on N with replacement.

However, in many situations the draw is not replaced. In this case you get the hyper-geometric distribution.

The function is given by:

Prob(r successes in n draws out of N) = RCr/[N-RCn-r * NCn]

With the binomial distribution the probability of success remains constant (=R/N) throughout. With the hypergeometric, the numerator for success reduces by one after each successful outcome whereas the denominator reduces by one whatever the outcome.