Table 3

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

EffectPotential outcome notationDescription
Controlled direct effect Embedded Image Effect of changing the exposure, with the mediator fixed at a specific level (m).
Natural direct effect22
Pure direct effect37 		Embedded Image 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 		Embedded Image 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 Embedded Image 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 Embedded Image 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.