Poisson Distribution
Also called: Poisson, rare event distribution
The Poisson distribution gives the probability of a number of events in a fixed interval when they occur independently at a constant average rate. Its defining feature is that mean and variance are equal, which makes it the natural model for rare events: operational-loss counts, defaults in a portfolio, claims arriving at an insurer. Real data is often overdispersed relative to it, because events cluster.
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