Fig. 1.
Elements of directed acyclic graphs. (A) A causal pathway between the exposure (X) and the outcome (Y). X influences Y through its effect on the mediator (M). (B) A noncausal pathway between X and Y due to a shared cause (the confounder-C). (C) A closed pathway between X and Y due to a shared effect (the collider-Z). (D) A complete causal graph of these pathways. The causal relationship of interest (blue pathway) cannot be quantified because of confounding (red pathway). The green pathway does not bias the relationship because it closed by the collider (Z).