Some metrical issues with meta-analysis of therapy effects

Two metrical issues, both concerning the index of effect size used in meta-analyses of therapy effects, are discussed with reference to such analyses. The first issue refers to use of the standard deviation of change scores as a metrical unit, and the conclusion is that the standard deviation of unadjusted post-test scores should be used instead. The second issue concerns measurement errors, and it is argued that the metrical unit—irrespective of which kind of variability metric is chosen—should be corrected for such errors, hence expressing each therapy effect relative to an estimated true rather than observed unit. It is proposed that a set of metrical alternatives should be used in a meta-analysis, so that the analysis is performed for each alternative and the results are compared across the alternatives.