Estimating and testing mediation and moderation in within-subject designs

Analyses designed to detect mediation and moderation of treatment effects a re increasingly prevalent in research in psychology. The mediation question concerns the processes that produce a treatment effect. The moderation que stion concerns factors that affect the magnitude of that effect. Although a nalytic procedures have been reasonably well worked out in the case in whic h the treatment varies between participants, no systematic procedures for e xamining mediation and moderation have been developed in the case in which the treatment varies within participants. The authors present an analytic a pproach to these issues using ordinary least squares estimation.