Fig. 4.

Four count distributions: Poisson, zero-inflated Poisson (ZIP), negative binomial (NB), and zero-inflated negative binomial (ZINB). a Parameterization, mean, variance, and zero proportion of each of the four distributions. b–e Illustration of the probability mass functions of Poisson (b), ZIP (c), NB (d), and ZINB (e) distributions that all have mean equal to 1. The horizontal axis indicates each possible value, and the vertical axis indicates the probability of taking each possible value. For each distribution, the parameter values are listed on the top right, and the zero proportion is listed at the bottom