Be. Wampold et al., A METAANALYSIS OF OUTCOME STUDIES COMPARING BONA-FIDE PSYCHOTHERAPIES- EMPIRICALLY, ALL MUST HAVE PRIZES, Psychological bulletin, 122(3), 1997, pp. 203-215
This meta-analysis tested the Dodo bird conjecture, which states that
when psychotherapies intended to be therapeutic are compared, the true
differences among all such treatments are 0. Based on comparisons bet
ween treatments culled from 6 journals, it was found that the effect s
izes were homogeneously distributed about 0, as was expected under the
Dodo bird conjecture, and that under the most liberal assumptions, th
e upper bound of the true effect was about .20. Moreover, the effect s
izes (a) were not related positively to publication date, indicating t
hat improving research methods were not detecting effects, and (b) wer
e not related to the similarity of the treatments, indicating that mor
e dissimilar treatments did not produce larger effects, as would be ex
pected if the Dodo bird conjecture was false. The evidence from these
analyses supports the conjecture that the efficacy of bona fide treatm
ents are roughly equivalent.