Assessing the feasibility of retrospective cohort studies.

While most epidemiologic cohort studies are preceded by some sort of feasibility study, details of such prior investigations are rarely reported. Yet it is during such feasibility studies that critical decisions are made, such as site selection and definition of exposure. Here we present the details on one such feasibility study, conducted to determine the possibility of a cohort mortality study of workers exposed to ethylene oxide. Issues discussed include methods for estimating sample size and power, for estimating levels of exposure, and for assessing the adequacy of personnel records.