EDUCATION CORNER Classification of epidemiological study designs

In this article, I present a simple classification scheme for epidemiological study designs, a topic about which there has been considerable debate over several dec-ades. I will argue that when the individual is the unit of analysis and the disease outcome under study is dichotomous, then epidemiological study designs can best be classified according to two criteria: (i) the type of outcome under study (incidence or prevalence) and (ii) whether there is sampling on the basis of the outcome. This classification system has previously been proposed by Greenland and Morgenstern (1988)1 and Morgenstern and Thomas (1993),2 all of whom followed previous authors3,4 in rejecting direc-tionality (i.e. prospective/retrospective or from expos-ure to outcome vs from outcome to exposure) as a ke