SAMPLE-SIZE CALCULATIONS FOR SINGLE GROUP POSTMARKETING COHORT STUDIES

In pharmacoepidemiology, single group cohort is the most frequently pr oposed design to determine if the incidence rate of an adverse drug re action among the exposed differs from a reference value. In many situa tions, the number of events expected in the cohort is too small to con duct sample size calculations based on the normal distribution. This p aper proposes, for a single group cohort study, calculations and table s derived from the Poisson distribution. The results are based on a on e-sided test with a 0.05 significance level and a power of 0.9 and 0.8 . Two parameters have to be specified a priori: the expected incidence of the event under the null hypothesis and the minimum risk ratio to be detected. The required sample size and the critical number of event s to reject the null hypothesis are directly derived from the tables. Results show that the normal approximation may lead to an underestimat ion of the required sample size.