Confounding and Interaction

All epidemiologic studies are (or should be) based on a particular source population followed over a particular risk period. The goal is usually to estimate the effect of one or more exposures on one or more health outcomes. When we are estimating the effect of a specific exposure on a specific health outcome, confounding can be thought of as a mixing of the effects of the exposure being studied with the effect(s) of other factor(s) on the risk of the health outcome of interest. Interaction can be thought of as a modification, by other factors, of the effects of the exposure being studied on the health outcome of interest, and can be subclassified into two major concepts: biological dependence of effects, also known as synergism; and effect-measure modification, also known as heterogeneity of a measure. Both confounding and interaction can be assessed by stratification on these other factors (i.e. the potential confounders or effect modifiers). The present chapter covers the basic concepts of confounding and interaction and provides a brief overview of analytic approaches to these phenomena. Because these concepts and methods involve far more topics than we can cover in detail, we provide many references to further discussion beyond that in the present handbook, especially to relevant chapters in Modern Epidemiology by Rothman and Greenland (1998).