Compound Poisson distribution

Definition

Suppose that

i.e., N is a random variable whose distribution is a Poisson distribution with expected value λ, and that

are identically distributed random variables that are mutually independent and also independent of N. Then the probability distribution of the sum of N i.i.d. random variables conditioned on the number of these variables ( N ):

has a well-defined distribution. In the case N = 0, then the value of Y is 0, so that then Y | N = 0 has a degenerate distribution.

The compound Poisson distribution is obtained by marginalising the joint distribution of (Y,N) over N, where this joint distribution is obtained by combining the conditional distribution Y | N with the marginal distribution of N.

Properties

Mean and variance of the compound distribution derive in a simple way from law of total expectation and the law of total variance. Thus

giving

Then, since E(N)=Var(N) if N is Poisson, and dropping the unnecessary subscripts, these formulae can be reduced to

The probability distribution of Y can be determined in terms of characteristic functions:

and hence, using the probability-generating function of the Poisson distribution, we have

An alternative approach is via cumulant generating functions:

Via the law of total cumulance it can be shown that, if the mean of the Poisson distribution λ = 1, the cumulants of Y are the same as the moments of X₁.

It can be shown that every infinitely divisible probability distribution is a limit of compound Poisson distributions. And compound Poisson distributions is infinitely divisible by the definition.

Discrete compound Poisson distribution

When X 1 , X 2 , X 3 , … are non-negative integer-valued i.i.d random variables with P ( X 1 = k ) = α k ,   ( k = 1 , 2 , … ) , then this compound Poisson distribution is named discrete compound Poisson distribution (or stuttering-Poisson distribution) . We say that the discrete random variable Y satisfying probability generating function characterization

has a discrete compound Poisson(DCP) distribution with parameters ( α 1 λ , α 2 λ , … ) ∈ R ∞ ( ∑ i = 1 ∞ α i = 1 , α i ≥ 0 , λ > 0 ) , which is denoted by

Moreover, if X ∼ DCP ( λ α 1 , … , λ α r ) , we say X has a discrete compound Poisson distribution of order r . When r = 1 , 2 , DCP becomes Poisson distribution and Hermite distribution, respectively. When r = 3 , 4 , DCP becomes triple stuttering-Poisson distribution and quadruple stuttering-Poisson distribution, respectively. Other special cases include: shiftgeometric distribution, negative binomial distribution, Geometric Poisson distribution, Neyman type A distribution, Luria–Delbrück distribution in Luria–Delbrück experiment. For more special case of DCP, see the reviews paper and references therein.

Feller's characterization of the compound Poisson distribution states that a non-negative integer valued r.v. X is infinitely divisible if and only if its distribution is a discrete compound Poisson distribution. It can be shown that the negative binomial distribution is discrete infinitely divisible, i.e., if X has a negative binomial distribution, then for any positive integer n, there exist discrete i.i.d. random variables X₁, ..., X_n whose sum has the same distribution that X has. The shift geometric distribution is discrete compound Poisson distribution since it is a trivial case of negative binomial distribution.

This distribution can model batch arrivals (such as in a bulk queue ). The discrete compound Poisson distribution is also widely used in actuarial science for modelling the distribution of the total claim amount.

When some α k are non-negative, it is the discrete pseudo compound Poisson distribution. We define that any discrete random variable Y satisfying probability generating function characterization

has a discrete pseudo compound Poisson distribution with parameters ( λ 1 , λ 2 , … ) =: ( α 1 λ , α 2 λ , … ) ∈ R ∞ ( ∑ k = 1 ∞ α k = 1 , ∑ k = 1 ∞ | α k | < ∞ , α k ∈ R , λ > 0 ) .

Other special cases

If the distribution of X is either an exponential distribution or a gamma distribution, then the conditional distributions of Y | N are gamma distributions in which the shape parameters are proportional to N. This shows that the formulation of the "compound Poisson distribution" outlined above is essentially the same as the more general class of compound probability distributions. However, the properties outlined above do depend on its formulation as the sum of a Poisson-distributed number of random variables. The distribution of Y in the case of the compound Poisson distribution with exponentially-distributed summands can be written in an form.

Compound Poisson processes

A compound Poisson process with rate λ > 0 and jump size distribution G is a continuous-time stochastic process { Y ( t ) : t ≥ 0 } given by

where the sum is by convention equal to zero as long as N(t)=0. Here, { N ( t ) : t ≥ 0 } is a Poisson process with rate λ , and { D i : i ≥ 1 } are independent and identically distributed random variables, with distribution function G, which are also independent of { N ( t ) : t ≥ 0 } .

For the discrete version of compound Poisson process, it can be used in survival analysis for the frailty models.

Applications

A compound Poisson distribution, in which the summands have an exponential distribution, was used by Revfeim to model the distribution of the total rainfall in a day, where each day contains a Poisson-distributed number of events each of which provides an amount of rainfall which has an exponential distribution. Thompson applied the same model to monthly total rainfalls.