poisson distribution

Discrete random variable distribution that expresses probabilities concerning the number of events per unit time. Unlike normal distribution, it is not symmetrical but instead is skewed to the left of the median. Good for inspection sampling, it is employed where the probability of an event is small and the number of opportunities for the event is large, such as the number of misprints in a book. Poisson distribution is an extension of binomial distribution and can be used as its approximation. One of its unusual properties is that its standard deviation equals the square root of the mean. Discovered in 1837 by the French mathematician Siméon-Davis Poisson (1781-1840).