Object - Cohort studies in pharmacoepidemiology can result in a unique
type of study, where subjects have complex types of exposure to drugs
(with periods of non-exposure as well). The object of this paper is t
o explain how to calculate the sample size of such a study. Method - I
t is assumed that adverse events follow Poisson distributions in the t
wo study groups. The null hypothesis is that the two groups have equal
rates of disease. Formulae are provided to calculate the sample size
required to significantly reject the null hypothesis. Sample size is g
iven as the number of events, rather than the number of subjects enter
ed. In a Poisson study, it is the ratio of the amount of person-years
exposure in the two groups that is important to calculate sample size,
rather than the actual amounts of exposure (or number of subjects in
the study). Some examples are included. (C) 1997 John Wiley & Sons, Lt
d.