Trials and Tribulations in the Meta-Analysis of Treatment Differences: Comment on Wampold et al. (1997)

A fair test of the Dodo bird conjecture that different psychotherapies are equally effective would entail separate comparisons of every pair of therapies. A meta-analysis of overall effect size for any particular set of such pairs is only relevant to the Dodo bird conjecture when the mean absolute value of differences is 0. The limitations of the underlying randomized clinical trials and the problem of uncontrolled causal variables make clinically useful treatment differences unlikely to be revealed by such heterogeneous meta-analyses. To enhance implications for practice, the authors recommend an intensified focus on patient-treatment interactions, cost-effectiveness variables, and separate metaanalyses for each pair of treatments.