Confounding and Confounder Control

Introduction

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, and important in causal inference (see Causation and Causal Inference). 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 upon stratification or adjustment for extraneous factors (a phenomenon called noncollapsibility or Simpson’s paradox;see Simpson’s Paradox; Collapsibility). In a third usage, originating in the experimental-design literature, confounding refers to inseparability of main effects and interactions under a particular design (see Interaction).

The three concepts are closely...