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For the cluster randomized trial, the treatment, implemented at the cluster level, may be hypothesized to impact a final outcome of interest via a mediating variable, with the mediator and outcome being measured typically at the participant level. Two opposing views—one that allows the mediation process to flow through a participant-level mediator and one that does not—have been presented in the literature. We integrate these perspectives and argue that it is the theoretical model and the nature of the mediating variable that determines whether the treatment effect may be mediated by a participant-level or only a cluster-level mediator in this design. An accompanying simulation study shows that when the mediation process is unnecessarily restricted to the cluster level, the power to detect the presence of mediation is substantially reduced. [ABSTRACT FROM PUBLISHER]

A Monte Carlo study extended the research of MacKinnon, Lockwood, Hoffman, West, and Sheets (2002) for single-level designs by examining the statistical performance of four methods to test for mediation in a multilevel experimental design. The design studied was a two-group experiment that was replicated across several sites, included a single intervening variable and outcome, and assumed that the effects of the treatment and mediator were constant across sites. The findings provide new evidence of the benefits of and further support for using the asymmetric confidence limits approach to test for mediation. In addition, the authors provide further support for using confidence intervals to assess if treatment effects are completely mediated, as using traditional hypothesis testing may lead to erroneous conclusions. [ABSTRACT FROM AUTHOR]