To the Editor: In their meta-analysis on the effectiveness of LTPP, Drs Leichsenring and Rabung1 reported evidence that LTPP is an effective treatment for complex mental disorders. Although the article addressed several limitations of the findings, other limitations should be considered as well.
First, Leichsenring and Rabung formulated an unconventionally broad research question. Systematic review objectives typically define the patient population, intervention, control treatment, outcomes, and study designs of interest.2 These authors defined the patient population broadly as a group of adults with mental disorders. The sole criterion that outcomes should have fulfilled is that they were reliable. Control treatments were not defined at all. Thus, analyzed diagnoses, outcomes, and control groups showed a very large clinical heterogeneity. Although the reviewers tried to account for this through subgroup analyses, their method of building clusters of heterogeneous disorders and outcomes still allowed for considerable variation. Thus, it is unclear for what disorder, for what outcome variables, and in comparison with which control groups the evidence was shown.