The negative binomial distribution is the probability distribution of the number of Bernoulli (yes/no) trials required to obtain r successes. Contrast it with the binomial distribution - the probability of x successes in n trials. Also with the Poisson distribution - the probability distribution of the number of events in a given time or space, in which the rate of event occurrence is constant. The negative binomial distribution is widely used in the analysis of count data whose distribution is over-dispersed, with the variance greater than the mean. For example, the distribution of plant or insect specimens, which is often clumped. This has been facilitated by the introduction, since 2000, of many new software routines for count data modeling.