FAQ/powposi32017-01-05 16:07:57PeterWatson22017-01-05 16:07:40PeterWatson12017-01-05 16:07:27PeterWatsonA quick method for performing power analyses on Poisson counts[From Thom Baguley] More in Thom's book at . Chapter 17 focuses on generalized linear models (mainly Poisson and logistic regression). As far as a quick and dirty sample size calculation goes you could take the square root of the counts and use a normal approximation* (i.e., the method that you'd use for regular ANOVA or t tests). For anything fancy it is more elegant to use simulation - but there will not be much gain from the quick and dirty approach unless you have a lot of information about the problem. * The Poisson distribution has a single parameter that is both mean and variance - the square root transformation is well known as a (very) approximate normalizing transformation in this case.