## Quick Reference

Greater variability in repeat estimates of a population proportion than would be expected if the population had a binomial distribution. For example, suppose that *n* observations are taken on independent Bernoulli variables that take the value 1 with probability *p*, and the value 0 with probability 1−*p*. The mean of the total of the observations will be *np* and the variance will be *np*(1−*p*). However, if the probability varies from variable to variable, with overall mean *p* as before, then the variance of the total will now be>*np*(1−*p*). In the context of plant and animal populations, extra-binomial variation may be termed overdispersion. See also index of dispersion.

*Subjects:*
Probability and Statistics.

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