In a test where only two outcomes are possible, the negative Binomial distribution gives you the probability of the number of times one outcome occurs before the other outcome occurs a specified number of times: for example, when a coin is tossed, the number of heads that fall before you get two tails. A special case of this is the Poisson distribution, which is when you are waiting for the very first outcome.

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### Links:

- Wikipedia: Negative binomial distribution
- mathworld.wolfram.com: Negative Binomial Distribution