A didactic device for teaching epidemiology students how to anticipate the effect of a third factor on an exposure-outcome relation.

Visualizing a three-dimensional relation among an exposure, an outcome, and a third factor is complicated, particularly when students are first being introduced to epidemiologic methods. The heuristic device described below can be used by teachers of epidemiology to help introductory students understand how a third factor will affect the association between an exposure and an outcome. The device is demonstrated using examples which assess the effects of potential confounding factors. However, it can be more generally applied to anticipate the effects of differential attrition, misclassification, and selection.