Overdispersion is when there is greater variability than there 'ought' to be in the data. Eg, the variance of counts is often greater than the mean, whereas the variance of a Poisson should equal the mean.

Overdispersion is when there is greater variability than there 'ought' to be in the data. For example, the variance of a set of counts is often greater than the mean, whereas the variance of a Poisson should equal the mean.