Fig. 4.

Possible relationships among the exposure (X), outcome (Y), and two ordered mediators (M₁ and M₂). (A) Only the direct effect from the exposure to the outcome is present; neither M₁ nor M₂ has a mediating role. (B) The indirect effect of the exposure is mediated by M₁ only. (C) The indirect effect of the exposure is mediated by M₂ only. (D) The indirect effect of the exposure is mediated by M₁, followed by M₂. The case where the indirect effect of the exposure is mediated by M₂ followed by M₁ is not displayed. In all panels, the solid line stands for the presence of a relationship while the dot line stands for the absence of a relationship. Here, we ignore the residual terms that are shown in Fig. 3.