Compound mixed Poisson distribution

Let the random sum Y=X_1+X_2+ \cdots +Y_N be the aggregate claims generated in a fixed period by an independent group of insureds. When the number of claims N follows a Poisson distribution, the sum Y is said to have a compound Poisson distribution. When the number of claims N has a mixed Poisson distribution, the sum Y is said to have a compound mixed Poisson distribution. A mixed Poisson distribution is a Poisson random variable N such that the Poisson parameter \Lambda is uncertain. In other words, N is a mixture of a family of Poisson distributions N(\Lambda) and the random variable \Lambda specifies the mixing weights. In this post, we present several basic properties of compound mixed Poisson distributions. In a previous post (Compound negative binomial distribution), we showed that the compound negative binomial distribution is an example of a compound mixed Poisson distribution (with gamma mixing weights).

In terms of notation, we have:

  • Y=X_1+X_2+ \cdots +Y_N,
  • N \sim Poisson(\Lambda),
  • \Lambda \sim some unspecified distribution.

The following presents basic proeprties of the compound mixed Poisson Y in terms of the mixing weights \Lambda and the claim amount random variable X.

Mean and Variance

\displaystyle E[Y]=E[\Lambda] E[X]

\displaystyle Var[Y]=E[\Lambda] E[X^2]+Var[\Lambda] E[X]^2

Moment Generating Function

\displaystyle M_Y(t)=M_{\Lambda}[M_X(t)-1]

Cumulant Generating Function

\displaystyle \Psi_Y(t)=ln M_{\Lambda}[M_X(t)-1]=\Psi_{\Lambda}[M_X(t)-1]

Measure of Skewness
\displaystyle E[(Y-\mu_Y)^3]=\Psi_Y^{(3)}(0)

\displaystyle =\Psi_{\Lambda}^{(3)}(0) E[X]^3 + 3 \Psi_{\Lambda}^{(2)}(0) E[X] E[X^2]+\Psi_{\Lambda}^{(1)}(0) E[X^3]

\displaystyle =\gamma_{\Lambda} Var[\Lambda]^{\frac{3}{2}} E[X]^3 + 3 Var[\Lambda] E[X] E[X^2]+E[\Lambda] E[X^3]

Measure of skewness: \displaystyle \gamma_Y=\frac{E[(Y-\mu_Y)^3]}{(Var[Y])^{\frac{3}{2}}}

Previous Posts on Compound Distributions

An introduction to compound distributions
Some examples of compound distributions
Compound Poisson distribution
Compound Poisson distribution-discrete example
Compound negative binomial distribution

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