An example of a probability distribution of a Poisson random variable is shown here. The Poisson distribution is often used in operations research. Heuristically speaking, this distribution is appropriate in many situations where an "event" occurs over a period of time, like the arrival of a customer; when it is as likely that this "event" will occur in one interval as in any other; also the occurrence of an event has no effect on whether or not another occurs. Then the number of customer arrivals in a fixed time is often assumed to have a Poisson distribution. Similarly, the demand for a given product is also often assumed to have this distribution.