Table 3

Different types of direct and indirect effects defined using potential outcome notation

 Effect Potential outcome notation Description Controlled direct effect Effect of changing the exposure, with the mediator fixed at a specific level (m). Natural direct effect22 Pure direct effect37 Effect of changing the exposure, with the mediator fixed at whatever (counterfactual) value it would have if the exposure were absent.* Natural indirect effect22 Total indirect effect37 Effect of changing the mediator between the values it would have with and without the exposure, with the exposure status fixed at exposed.* Pure indirect effect37 Effect of changing the mediator between the values it would have with and without the exposure, with the exposure status fixed at unexposed.* Total direct effect37 Effect of changing the exposure, with the mediator fixed at whatever (counterfactual) value it would have if the exposure were present.*24
• *Requires ‘cross-world’ independence assumption. See Ref. 24.