The Poisson distribution derives from the Binomial distribution to cover those situations in which the population in which "events" occur tends to infinity, while the product of the population and the mean remains finite, so that the probability of occurrence of an "event" is extremely small. The use of this distribution facilitates the calculation of the probability of either 0 or 1 or 2 or .... occurrences of an "event" within a given period of time. This makes the Poisson distribution particularly useful for the calculation of Line Replaceable Unit (LRU) spares.

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