The assessment of mediation is important for testing the mechanisms that explain an observed relationship between exposure and disease. Several types of direct and indirect effects have been defined, broadly characterized as either controlled or natural. The identification of these effects requires a stricter set of assumptions than those necessary for the identification of the total effect of exposure on disease. The particular assumptions that are required differ depending on the type of effect. We use an approach based on response types to derive new assumptions for the identification of direct and indirect effects, both controlled and natural. These assumptions are stated in terms of response types and potential outcomes, and are compared with those already in the literature. This approach yields an alternative, and sometimes less stringent, set of assumptions for the identification of direct and indirect effects than those previously proposed.
From the aDepartment of Epidemiology, Mailman School of Public Health, Columbia University, New York, NY; bDepartment of Psychiatry, Western Psychiatric Institute and Clinic, University of Pittsburgh Medical Center, Pittsburgh, PA; and cDepartments of Epidemiology and Biostatistics, Harvard School of Public Health, Boston, MA.
Submitted 24 March 2008; accepted 29 May 2009; posted 25 May 2011.
Supported by Medical Scientist Training Program and Department of Epidemiology, Columbia University.
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Correspondence: Danella M. Hafeman, Department of Psychiatry, 3811 O'Hara Street, Pittsburgh, PA 15213. E-mail: firstname.lastname@example.org.