The Poisson distribution

This discrete distribution gives the probability distribution of the number of times something happens in a given period, if the things happen randomly at a given average rate.

If a random variable X is distributed in this way with a mean of λ we say X ~ Po(λ)

λ (lambda), the greek letter l, is for historic reasons inextricably linked with the Poisson distribution!

The probability of r arrivals in the same period is -mathformula2 not MarkupType-
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