Observational epidemiology refers to the branch of epidemiology devoted to using nonexperimental [Page 758]studies to describe the health status of populations and generate evidence about determinants of health outcomes. Experimental designs in epidemiology, generally referred to as clinical trials, involve assignment of the principal independent variable, the ‘treatment,’ to subjects. Often this assignment is randomly allocated, which offers profound advantages for making causal inferences, but it can also be nonrandom. In observational studies, the investigators do not assign treatment to subjects. The principal independent variable is some endogenous or exogenous exposure observed as it naturally occurred. When observational epidemiologic studies are designed to draw inferences about health outcome across different exposure groups, they are considered ‘analytic.’ When they are intended only to describe the frequency ...