The Dodo bird verdict is alive and well - Mostly
We examined 17 meta-analyses of comparisons of active treatments with each other, in contrast to the more usual comparisons of active treatments with controls. These meta-analyses yielded a mean uncorrected absolute effect size for Cohen's d of .20, which is small and nonsignificant (an equivalent Pearson's r would be.10). The smallness of this effect size confirms Rosenzweig's supposition in 1936 about the likely results of such comparisons. In the present sample, when such differences were corrected for the therapeutic allegiance of the researchers involved in comparing the different psychotherapies, these differences tend to become even further reduced in size and significance, as shown previously by Luborsky, Diguer, Seligman, et al. (1999). © 2002 American Psychological Association D12.
Clinical Psychology: Science and Practice
Luborsky, L., Rosenthal, R., Diguer, L., Andrusyna, T., Berman, J., Levitt, J., Seligman, D., & Krause, E. (2002). The Dodo bird verdict is alive and well - Mostly. Clinical Psychology: Science and Practice, 9 (1), 2-12. https://doi.org/10.1093/clipsy/9.1.2