The probability for a vector x of non-negative integers x1, x2, ... xLength[p] in a negative multinomial distribution is proportional to (n-1+xi)!(pixi/xi!).
The parameter n can be any positive real number, and p can be any vector of nonnegative real numbers that sum to less than unity.
If n is a positive integer, NegativeMultinomialDistribution[n, p] gives the distribution of the failure counts in a sequence of trials with success probability 1-Total[p] and Length[p] types of failure before n successes occur.
NegativeMultinomialDistribution can be used with such functions as Mean, CDF and RandomInteger.
xi)!
(pixi/xi!).
EXAMPLES
Basic Examples 
Scope 