Population-based Cohort Studies

In population-based cohort studies, a sample, or even the entirety, of a defined population is selected for longitudinal assessment of exposure-outcome relations. Because of their typically high cost and logistic complexities, population-based cohort studies generally evaluate multiple hypotheses, some defined a priori as well as some suggested in the course of the study either based on interim analyses or on advances in the field, particularly when data are repeatedly collected on cohort members. Perhaps the most often cited justification for conducting a population-based study is its external validity—that is, the applicability of its results to a defined population. The study of a cohort which is representative of a defined population offers three additional advantages. 1) It allows the estimation of distributions and prevalence rates of relevant variables in the reference population (information on risk factors is used for the calculation of population attributable risks) (1). 2) Risk factor distributions measured at baseline in a study involving periodic examinations of the cohort can be compared with distributions in future crosssectional samples so as to assess risk factor trends over time (2). 3) A representative sample is the ideal setting in which to carry out unbiased evaluations of relations, not only of confounders to exposures and outcomes, but also among any other variables of interest, even those which were not specified in the original study hypotheses. This review will briefly summarize some key features of population-based cohort studies, and then the main rationale for conducting these studies, that is external validity, will be discussed.