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Causal Mediation Analysis is concerned with distinguishing different causal pathways that may link a treatment and an outcome.
In the simplest case we may wish to know whether a treatment acts directly on the outcome or via a mediator.

In this talk I will first describe a new conceptual framework that is based on decomposing the treatment into sub-components.

I will then contrast this approach to a non-interventionist approach advocated by Pearl that is based on nested counterfactuals.

The  new interventionist approach has several advantages over the counterfactual approach:

  • It does not require the existence of well-defined interventions or counterfactuals on mediators;
  • Identified effects are, in principle, empirically testable via interventions;
  • The new theory preserves the dictum “no causation without manipulation”;
  • It replaces the current complex definitions of path specific effects as nested counterfactuals with easily understood definitions in terms of concrete experimental interventions;
  • The new approach facilitates communication with subject matter experts.

However, notwithstanding these differences, when both are identified from data,  the identifying formulae under an interventionist view and the non-interventionist view are identical, even though the effects are different.

This talk is joint work James M. Robins (Harvard School of Public Health) and Ilya Shpitser (Johns Hopkins University).