In: Statistics and Probability
Let N1 , N2 , N3 follow a trinomial distribution with parameters n, assume that n follows a Poisson distribution with parameter λ > 0. Also assume that, conditionally on N, the random variables N1, N2, N3 follow a trinomial distribution with N trials and category probabilities p1, p2, p3 with p1 + p2 + p3 = 1. Compute the covariance and correlation of (N1,N2)
The pmf of multinomial random variable 
 is defined as
where
 This is sampling with replacement:
The Covariance 
The variance 
.
The correlation is

We see the correlation is independent of 
For trinomial distribution, 
. and conditional on 
,

Since 
, 
.
The expected/ unconditional Covariance is

The conditional correlation is 
.
The unconditional correlation is,
