| Abstract: | Comparative studies of psychotherapy often find few or no differences in the outcomes that alternative treatments produce. Although these findings may reflect the comparability of alternative treatments, studies are often not sufficiently powerful to detect the sorts of effect sizes likely to be found when two or more treatments are contrasted. The present survey evaluated the power of psychotherapy outcome studies to detect differences for contrasts of two or more treatments and treatment vs no-treatment. 85 outcome studies were drawn from 9 journals over a 3-yr period (1984–1986). Data in each article were examined first to provide estimates of effect sizes and then to evaluate statistical power at posttreatment and follow-up. Findings indicate that the power of studies to detect differences between treatment and no treatment is quite adequate given the large effect sizes usually evident for this comparison. However, the power is relatively weak to detect the small-to-medium effect sizes likely to be evident when alternative treatments are contrasted. Thus, the equivalent outcomes that treatments produce may be due to the relatively weak power of the tests. Implications for interpreting outcome studies and for designing comparative studies are highlighted. (PsycINFO Database Record (c) 2010 APA, all rights reserved) |
