Mediators and moderators in meta-analysis: There’s a reason we don’t let dodo birds tell us which psychotherapies should have prizes

In primary studies, psychotherapy researchers frequently search for mediator and moderator vari-ables that can help them understand the relationship between treatment and outcome. Yet a review of past psychotherapy meta-analyses r vealed that none examined the possible role of mediator variables; and although all of them searched for moderators of study outcome, that search was generally not as complete as it could have been. This article illustrates methods for studying such mediator and moderator variables in meta-analysis, discusses their advantages and disadvantages, and shows how the inclusion of these variables can change interpretation fmeta-analytic results. In particular, the perennial interpretation of past psychotherapy meta-analyses that therapeutic orientation makes no difference to outcome--or asthe dodo bird put it: "Everyone has won and all must have prizes'--may be wrong. Orientation may make significant difference, but only by virtue of its moderating and mediating effects. To the best of our knowledge, all meta-analyses ver done have concluded that (on the average) clients receiving psycho-therapy do better than clients not receiving psychotherapy. In fact, the computation of average therapy effects over studies i