COMMENTARY OF “MEDIATION ANALYSIS WITHOUT SEQUENTIAL IGNORABILITY: USING BASELINE COVARIATES INTERACTEDWITH RANDOM ASSIGNMENT AS INSTRUMENTAL VARIABLES” BY DYLAN SMALL

Abstract

I applaud Dr. Small for advancing causal mediation analysis and thank the editors for the opportunity
to comment on this valuable article. Small’s project (Small, 2012) was to relax and test the
assumptions on which a previously proposed model relies; in the second half of this discussion I will
assess those assumptions and others on which the model hinges. But first I will review the various
schools of mediation analysis and situate the estimand considered by Small within the somewhat
esoteric domain of mediation estimands.