# Zero-Modified Distribution

A zero-modified distribution is a probability distribution that starts at zero. More specifically, it is a distribution modified to put extra probability mass at zero.

Any distribution of this type has the “zero-modified” prefix; they are commonly used to model data that has a high occurrence of zero counts.

A similar distribution is the zero-truncated distribution. the difference is that the the probability of zero occurring in a zero-truncated distribution is 0:

• Zero-modified distribution: p0 > 0
• Zero-truncated distribution p0 = 0.

All zero-truncated distributions are members of the zero-modified family.

## Zero-modified counting distribution

Counting distributions, such as the Poisson distribution, are discrete distributions that have a domain of only non-negative integers {1, 2, 3, …). These types of distribution are often used to model the number of event occurrences such as the number of hurricanes in a calendar year.

Counting distributions cannot be used to model events that include zero. For example, if zero hurricanes happen in a year, the Poisson distribution — without modification — is not suitable. That’s because probabilities are always greater than 0 — never equal to zero. Thus, one option is to modify the distribution to include zero in the domain — giving a mixture model of a standard Poisson distribution and a degenerate distribution at zero.

For example, the probability mass function (PMF) of a zero-modified Poisson distribution is p(0) = p0 and: