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To the Editor: Drs Leichsenring and Rabung¹ reported that long-term psychodynamic psychotherapy (LTPP) is more effective than shorter forms of psychotherapy for complex mental disorders based on a between-group effect size of 1.8 from 7 comparative trials that they meta-analyzed. The authors did not indicate that they were concerned about this and other surprisingly large effect sizes they reported.

Between-group effect sizes can be presented as group differences in terms of standard deviations or as point biserial correlations between group (eg, LTPP vs shorter-term therapies) and treatment effect. They are equivalent and convertible using a formula or tables.² The authors, however, apparently erroneously calculated within-group pre-post effect sizes and point biserial correlations between group and within-group effect sizes, which is altogether different. It seems that they converted these correlations between group and within-group pre-post effect sizes to produce deviation-based effect sizes that do not appear reasonable.

As a result, although . . . [Full Text of this Article]

Brett D. Thombs, PhD
brett.thombs@mcgill.ca

Marielle Bassel, BA; Lisa R. Jewett, BA
Department of Psychiatry
McGill University
Montreal, Quebec, Canada

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