The magical number .7 +/- .2: meta-meta-analysis of the probability of superior outcome in comparisons involving therapy, placebo, and control.
The "probability of superiority estimate" (PS) estimates the probability that a randomly sampled client from a population given a treatment will have an outcome that is superior to that of a randomly sampled client from a population given another treatment. The meta-analytic clinical outcome literature was examined to calculate mean PS (PS) for comparisons involving therapy versus control, therapy versus placebo, therapy versus therapy, and placebo versus control. The range of PS was found to be approximately .7 +/- .2, with median PS greatest when therapy and control are compared (Mdn PSTC = .70, where T = therapy and C = control) and least when 2 therapies are compared (Mdn PSTT = .56). Results suggested that there is more to therapeutic success than placebo effects (Mdn PSTP = .66, where T = therapy and P = placebo) and that placebo is typically better than do-nothing control conditions (Mdn PSPC = .62). The present exceptionally large study, controlling for dependencies and confounding variables, may put to rest the question of the superiority of therapy to placebo. It also appears that the strength of effect of therapy is typically at least average among the effects of independent variables in psychology.