A mediator is a variable that explains, totally or partially, the relationship between a predictor and an outcome. In other words, a mediating variable is a mechanism through which a predictor exerts its effect on an outcome variable. Mediation is important in epidemiology because health events are rarely due to direct causes. For instance, low socioeconomic condition increases the risk of low birthweight (LBW) through a complex mechanism mediated by food supply to the pregnant woman and weight gain during pregnancy. If the predictor is an intervention, identifying mediating variables is essential to understand how some actions produce certain outcomes.

Given any two correlated variables, X and Y, and no outside theoretical information, it is impossible to say whether changes in X cause changes in Y, ...