title: Methods to select results to include in meta-analyses deserve more consideration in systematic reviews.
abstract: OBJECTIVE
  To investigate how often systematic reviewers encounter multiple trial effect estimates that are available for inclusion in a particular meta-analysis (multiplicity of results) and the methods they use to select effect estimates.
  
  
  METHODS
  We randomly sampled Cochrane and MEDLINE-indexed non-Cochrane reviews published between January 2010 and January 2012. The first presented meta-analysis of an effect measure for a continuous outcome in each review was identified, and methods to select results to include in this meta-analysis were extracted from review protocols and reviews. All effect estimates that were available for inclusion in the meta-analyses were extracted from trial reports.
  
  
  RESULTS
  We examined 44 reviews. Multiplicity of results was common, occurring in 49% of trial reports (n = 210). Prespecification of decision rules to select results from multiple measurement scales and intervention/control groups (in multi-arm trials) was uncommon (19% and 14% of 21 review protocols, respectively). Overall, 70% of reviews included at least one randomized controlled trial with multiplicity of results, but this occurred less frequently in reviews with a protocol (risk difference, -25%; 95% confidence interval: -52%, 1%).
  
  
  CONCLUSIONS
  Systematic reviewers are likely to encounter multiplicity of results in the included trials. We recommend that systematic reviewers always consider predefining methods to select results to include in meta-analyses. Methods focusing on selection of measurement scales and how to deal with multi-arm trials would be most valuable.
authors.affiliation: Australasian Cochrane Centre, School of Public Health and Preventive Medicine, Monash University, Level 1, 549 St Kilda Road, Melbourne, Victoria 3004, Australia. Electronic address: matthew.page@monash.edu.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Page
authors.affiliation: Australasian Cochrane Centre, School of Public Health and Preventive Medicine, Monash University, Level 1, 549 St Kilda Road, Melbourne, Victoria 3004, Australia.
authors.forename: Joanne E
authors.initials: JE
authors.lastname: McKenzie
authors.affiliation: National Trauma Research Institute, Central Clinical School, Monash University, 85-89 Commercial Road, Melbourne, Victoria 3004, Australia.
authors.forename: Marisa
authors.initials: M
authors.lastname: Chau
authors.affiliation: Australasian Cochrane Centre, School of Public Health and Preventive Medicine, Monash University, Level 1, 549 St Kilda Road, Melbourne, Victoria 3004, Australia.
authors.forename: Sally E
authors.initials: SE
authors.lastname: Green
authors.affiliation: Department of Epidemiology and Preventive Medicine, School of Public Health and Preventive Medicine, Monash University, Level 6, The Alfred Centre, 99 Commercial Road, Melbourne, Victoria 3004, Australia.
authors.forename: Andrew
authors.initials: A
authors.lastname: Forbes
mesh_terms: D016247:Information Storage and Retrieval; D015201:Meta-Analysis as Topic; D012107:Research Design; D012196:Review Literature as Topic
keywords: Bias; Meta-analysis; Randomized controlled trials; Reporting; Research methodology; Systematic review