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 none of the 7 studies had an overall effect size greater than 1.45, the authors reported a combined effect size of 1.8, which is statistically impossible. In a slightly larger set of 8 trials, the authors reported that LTPP had a larger overall effect than shorter-term therapies (0.96 vs 0.47) but a point biserial correlation (0.60) equivalent to a between-group effect size of 1.5. However, between-group effect sizes must be smaller than within-group effect sizes when both groups have positive effects. Similarly, these methods generated an implausible between-group effect size of 6.9 for personality functioning based on 3 trials,3 - 5 none of which reported an effect size for personality functioning larger than approximately 2.
In addition, we believe that this collection of studies was not suitable for meta-analysis. Each reviewed study had 15 to 30 patients in the LTPP treatment group. It seems unlikely that investigators would attempt to publish a negative study with so few patients (or that such a study would be accepted for publication), which means that all published studies would have an effect size of at least 0.50 to 0.75, the minimum for statistical significance. This is an artificial floor that guarantees a large effect when these studies are combined.
Financial Disclosures: None reported.
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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