An unknown number, say N, of animals inhabit a certain region.

To obtain some information about the population size, ecologists often perform the following experiment: They first catch a number, say r, of these animals, mark them in some manner, and release them. After allowing the marked animals time to disperse throughout the region, a new catch of size, say, n is made. Let X denote the number of marked animals in this second capture. If we assume that the population of animals in the region remained fixed between the time of the two catches and that each time an animal was caught it was equally likely to be any of the remaining uncaught animals, it follows that X is a hypergeometric random variable such that 

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P{X}= N-r (A) (N=7) (w) P;(N)