问题如下：
Gerard Kuper is modeling the number of operational risk loss events that could adversely impact Bank ABC in 2010. He expects the number of operational risk loss events for the year to be relatively small. Which type of distribution is he least likely to use?
选项： Normal distribution
Binomial distribution
C.Negative binomial distribution
D.Poisson distribution
解释：
A is correct. The last three distributions require the number n to be positive, which is not the case for the normal distribution.
请问Negative binomial distribution指怎样的分布呢？
