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 variables that can help them understand the relationship between treatment and outcome. Yet a review of past psychotherapy meta-analyses revealed 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 of meta-analytic results. In particular, the perennial interpretation of past psychotherapy meta-analyses that therapeutic orientation makes no difference to outcome--or as the 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.