The Hypergeometric Distribution

We have seen that the binomial distribution is based on the hypothesis of an infinite population N, a condition that can be practically realized by sampling from a finite population with replacement.

If this does not occur, meaning if we are sampling from a population without replacement, we must use the hypergeometric distribution. (In reality, if N is large, the hypergeometric probability density function tends towards the binomial).

The hypergeometric distribution is used to calculate the probability of obtaining a certain number of successes in a series of binary trials (yes or no), which are dependent and have a variable probability of success.

The hypergeometric distribution allows us to answer questions like:

If I take a sample of size N, in which M elements meet certain requirements, what is the probability of drawing x elements that meet those requirements?

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The Negative Binomial Distribution (or Pascal Distribution)

The negative binomial distribution describes the number of trials needed to achieve a certain number of successes in a series of independent trials. For example, it answers the question: flipping a balanced coin (50% chance of heads on each flip), what is the probability that the third head comes up exactly on the fifth flip?

The negative binomial distribution is useful in many fields, including statistics, economics, biology, and physics. And also in “our” SEO.

(If instead we want the probability of a certain number of successes in a fixed number of trials, say three heads in five flips, the right tool is the binomial distribution: the negative binomial turns the question around.)

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