The word confounding has been used to refer to at least three distinct concepts. In the oldest and most widespread usage, confounding is a source of bias in, estimating causal effects. This bias is sometimes informally described as a mixing of effects of extraneous factors (called confounders) with the effect of interest. This usage predominates in nonexperimental research, especially in epidemiology and sociology. In a second and more recent usage originating in statistics, confounding is a synonym for a change in an effect measure on stratification or adjustment for extraneous factors (a phenomenon called noncollapsibility or Simpson's paradox). In a third usage, originating in the experimental-design literature, confounding refers to inseparability of main effects and interactions under a particular design. The three concepts are closely ...