A systematic review of core outcome set development studies demonstrates difficulties in defining unique outcomes

Amber E Young*, Sara T Brookes, Kerry N L Avery, Anna Davies, Chris Metcalfe, Jane M Blazeby

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

1 Citation (Scopus)
168 Downloads (Pure)


OBJECTIVES: Core outcome set (COS) development often begins with a systematic review to identify outcomes. Reviews frequently show heterogeneity in numbers of outcomes reported across trials. Contributing to this is a lack of a uniform definition for an outcome. This study proposes a first working definition for a unique trial outcome to support reporting a quantitative assessment of outcome reporting heterogeneity (ORH).

STUDY DESIGN AND SETTING: Eligible COS literature (development papers, protocols, and reviews) were identified using the COMET database, Ovid MEDLINE, and PubMed. Outcome numbers, definitions, timing, and grouping methodology were examined.

RESULTS: One hundred and thirty two studies were included. 82 (88.1%) studies (excluding protocols) reported a total number of unique outcomes (median, 82; range, 12-5776; IQR, 261). Timing of assessment was reported in 32 (31.4%) studies. Methods to group similar outcomes were reported in 8 (7.8%) articles. No study defined how outcomes were agreed as different and how final numbers of unique outcomes were determined. It is proposed that a unique outcome requires original meaning and context. Thus ORH is suggested to be the reporting of multiple unique outcomes across trials related to one health care condition.

CONCLUSION: This review identified inconsistencies in how authors define, extract, group, and count trial outcomes. Further work is needed to refine our proposed definitions to optimize COS development and allow a quantifiable measure of ORH.

Original languageEnglish
Pages (from-to)14-24
Number of pages11
JournalJournal of Clinical Epidemiology
Early online date2 Jul 2019
Publication statusPublished - 1 Nov 2019

Bibliographical note

Crown Copyright © 2019. Published by Elsevier Inc. All rights reserved.

Structured keywords

  • BTC (Bristol Trials Centre)
  • Centre for Surgical Research


  • Clinical trials
  • Core outcome set
  • Outcomes
  • Research methodology
  • Systematic review

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