Defining and estimating causal direct and indirect effects when setting the mediator to specific values is not feasible

TL;DR

This paper introduces 'organic' direct and indirect effects, enabling causal mediation analysis without fixing the mediator to specific values, addressing practical challenges in setting mediators in real-world scenarios.

Contribution

It proposes a novel framework for defining and estimating causal effects that do not require setting the mediator to specific values, broadening applicability.

Findings

Defines organic direct and indirect effects.

Provides methods for estimation without fixing mediators.

Applied example to HIV/AIDS treatment effects.

Abstract

Natural direct and indirect effects decompose the effect of a treatment into the part that is mediated by a covariate (the mediator) and the part that is not. Their definitions rely on the concept of outcomes under treatment with the mediator "set" to its value without treatment. Typically, the mechanism through which the mediator is set to this value is left unspecified, and in many applications it may be challenging to fix the mediator to particular values for each unit or individual. Moreover, how one sets the mediator may affect the distribution of the outcome. This article introduces "organic" direct and indirect effects, which can be defined and estimated without relying on setting the mediator to specific values. Organic direct and indirect effects can be applied for example to estimate how much of the effect of some treatments for HIV/AIDS on mother-to-child transmission of HIV-infection is mediated by the effect of the treatment on the HIV viral load in the blood of the mother.