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Definition: Gamma distribution is a distribution that arises naturally in processes for which the waiting times between events are relevant. It can be thought of as a waiting time between Poisson distributed events. Probability density function: The waiting time until the hth Poisson event with a rate of change Î» is.
1 if k is a positive integer, then Î³(k) = (k â 1)! is the gamma function.  Î¸ = 1 / Î» is the mean number of events per time unit, where Î» is the mean time between events. For example, if the mean time between phone calls is 2 hours, then you would use a gamma distribution with Î¸=1/2=0.5.