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To the Editor: The selective reporting of outcomes in randomized controlled trials (RCTs), or outcome reporting bias, has been identified,¹ but its prevalence and potential effect in meta-analyses is unknown. We investigated (1) to what extent Cochrane meta-analyses include only a proportion of identified RCTs when estimating treatment effects on patient-important outcomes and (2) whether the proportion of RCTs included in a meta-analysis is associated with its pooled effect size.

Methods

Issue 4 of the Cochrane Library, 2005, contains 2524 reviews. Analysis of the first 56 meta-analyses found 10 that met the eligibility criterion of containing 10 or more RCTs. A power calculation based on their regression coefficients indicated that detecting statistically significant coefficients of this magnitude required approximately 100 reviews. We therefore selected 500 reviews by random-number generator for assessment.

Two investigators independently identified reviews with 10 or more RCTs. For each review, 3 investigators selected 2 dichotomous and 2 . . . [Full Text of this Article]

Toshi A. Furukawa, MD, PhD
furukawa@med.nagoya-cu.ac.jp

Norio Watanabe, MD; Ichiro M. Omori, MD
Department of Psychiatry and Cognitive-Behavioral Medicine
Nagoya City University Graduate School of Medical Sciences
Nagoya, Japan

Victor M. Montori, MD, MSc
Knowledge and Encounter Research Unit
Department of Medicine
Mayo Clinic College of Medicine
Rochester, Minn

Gordon H. Guyatt, MD, MSc
Department of Clinical Epidemiology and Biostatistics
McMaster University
Hamilton, Ontario

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