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

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 presentation 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.

This presentation is based on two technical reports, which can be found on arxiv:

http://arxiv.org/abs/1509.00351: Defining and estimating causal direct and indirect effects when setting the mediator to specific values is not feasible, Judith J Lok

http://arxiv.org/abs/1510.02753: Organic direct and indirect effects with post-treatment common causes of mediator and outcome, Judith J Lok