Justification and reporting of subgroup analyses were lacking or inadequate in randomized controlled trials

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Objectives: The aim of the article was to assess the appropriateness and rationales of subgroup analyses planned in protocols of randomized controlled trials and reported in subsequent corresponding trial publications. Study Design and Setting: We searched PubMed to identify trial protocols published in journals during 2006–2017. From a total of 3,774 initially identified records, we included a random sample of 479 protocols and identified 280 trial publications corresponding to the included protocols. Results: Subgroup analyses were specified in 19% of the protocols and reported in 21% of the trial publications. Of the 94 protocols with planned subgroup analyses, 32% mentioned testing for interaction, and only three considered statistical power. Subgroup analyses were not prespecified in 56% of the 59 trial publications with subgroup analyses. Subgroup analyses were stated as prespecified in nine trial publications, without support evidence from the corresponding protocols. Subgroup analyses were often reported insufficiently for assessing the consistency of subgroup effects across studies. Justifications for subgroup analyses were provided in only four trial protocols and seven trial publications. Conclusion: Inappropriate specification and reporting of subgroup analyses remain problematic in protocols and reports of randomized controlled trials. Justifications or rationales for subgroup analyses were only rarely provided in trial protocols and reports.

Original languageEnglish
Pages (from-to)17-25
Number of pages9
JournalJournal of Clinical Epidemiology
Early online date15 Dec 2018
Publication statusPublished - Apr 2019


  • Clinical trial
  • Prespecification
  • Study protocol
  • Study reporting
  • Subgroup analysis
  • Subgroup effect

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