Methods in epidemiology: observational study designs.

This article is the first of a three-part series intended to enhance clinical pharmacists' understanding of methods frequently used in epidemiologic research and their applications. The basic tenets of epidemiology and uses for data derived from epidemiologic studies are given, along with a high-level overview of the differences between experimental and observational study designs. The defining characteristics of each of the observational study designs (case report or case series, ecologic, cross-sectional, cohort, case-control, nested case-control, and case-cohort) and the resultant strengths and limitations of the study designs are presented. Applications for observational studies in pharmacoepidemiology (including the case-crossover and case-time-control study designs) are discussed. Finally, points to consider when evaluating data from observational studies are addressed.