Abstract:
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Adopting the potential outcomes causal framework, this study extends weighting approaches to causal mediation analysis in multisite randomized trials. The goal is to inform program improvement by revealing not only the prevalent causal mechanism but also how the mechanism may vary across sites. We extend the Ratio-of-Mediator-Probability Weighting (RMPW) approach (Hong, 2010; Hong, Deutsch, & Hill, 2011) and the Inverse Probability Weighting (IPW) approach (Huber, 2014) to multisite trials in which a continuous mediator may be nonlinearly associated with the outcome. Moreover, the mediator-outcome relationship may vary across the treatment conditions. Primary interests are in the population average and between-site variation of the direct and indirect effects. To identify an optimal weighting procedure, we consider alternative ways of implementation and assess through Monte Carlo simulations their relative performance in comparison with multilevel path analysis/SEM. The study is motivated by a re-evaluation of Job Corps programs serving disadvantaged youths. We focus on the program impact on future earnings mediated by an individual's length of stay in a training program.
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