Sean F . Reardon and Stephen W . Raudenbush Average Causal Effects ? Under What Assumptions Do Site - by - Treatment Instruments Identify

The increasing availability of data from multisite randomized trials provides a potential opportunity to use instrumental variables (IV) methods to study the effects of multiple hypothesized mediators of the effect of a treatment. We derive nine assumptions needed to identify the effects of multiple mediators when using site-by-treatment interactions to generate multiple instruments. Three of these assumptions are unique to the multiple-site, multiplemediator case: (1) the assumption that the mediators act in parallel (no mediator affects another mediator); (2) the assumption that the siteaverage effect of the treatment on each mediator is independent of the site-average effect of each mediator on the outcome; and (3) the assumption that the site-by-compliance matrix has sufficient rank. The first two of these assumptions are nontrivial and cannot be empirically verified, suggesting that multiple-site, multiple-mediator IV models must be justified by strong theory. 1 Stanford University, Stanford, CA, USA 2 University of Chicago, Chicago, IL, USA Corresponding Author: Sean F. Reardon, 520 Galvez Mall, #526, Stanford University, Stanford, CA 94305, USA. Email: [email protected] Sociological Methods & Research 42(2) 143-163 a The Author(s) 2013 Reprints and permission: sagepub.com/journalsPermissions.nav DOI: 10.1177/0049124113494575 smr.sagepub.com at Stanford University Libraries on February 8, 2014 smr.sagepub.com Downloaded from