Figure 1.

Directed acyclic graph (DAG) for simple confounding by coexposure. Causal pathways are denoted by arrows and causal coefficients by $c_i$. In this example, only $X_1$ causes the outcome. The two exposures ($X_1$, $X_2$) have a common source (U), with causal coefficients omitted. $r_{12}$ is the bivariate correlation coefficient for $X_1$ and $X_2$.