In probability theory, a compound Poisson distribution is the probability distribution of a "Poisson-distributed number" of independent identically-distributed random variables. More precisely, suppose
i.e., N is a random variable whose distribution is a Poisson distribution with expected value λ, and
are identically distributed random variables that are mutually independent and also independent of N. Then the probability distribution of the sum
is a compound Poisson distribution. (When N = 0, then the value of Y is 0.)
In terms of the basic moments,
as E(N)=Var(N) if N is Poisson it can be reduced to
Via the law of total cumulance it can be shown that the moments of X1 are the cumulants of Y.
It can be shown that every infinitely divisible probability distribution is a limit of compound Poisson distributions.
Compound Poisson processes
A compound Poisson process with rate and jump size distribution G is a continuous-time stochastic process given by
where, is a Poisson process with rate , and are independent and identically distributed random variables, with distribution function G, which are also independent of
it:Distribuzione composta di Poisson