title: A methodological review of recent meta-analyses has found significant heterogeneity in age between randomized groups.
abstract: BACKGROUND
  There is evidence to suggest that component randomized controlled trials (RCTs) within systematic reviews may be biased. It is important that these reviews are identified to prevent erroneous conclusions influencing health care policies and decisions.
  
  
  OBJECTIVE
  To assess the likelihood of bias in trials in 12 meta-analyses.
  
  
  METHODS
  A review of 12 systematic reviews.
  
  
  METHODS
  Twelve recently published systematic reviews with 503 component randomized trials, published in the British Medical Journal, The Lancet, Journal of the American Medical Association, and The Annals of Internal Medicine before May 2012.
  
  
  METHODS
  Systematic reviews were eligible for inclusion if they included only RCTs. We obtained the full text for the component RCTs of the 12 systematic reviews (in English only). We extracted summary data on age, number of participants in each treatment group, and the method of allocation concealment for each RCT.
  
  
  RESULTS
  Five of the 12 meta-analyses exhibited heterogeneity in age differences (I(2) > 0.30), when there should have been none. In two meta-analyses, the age of the intervention group was significantly greater than that of the control group. Inadequate allocation concealment was a statistically significant predictor of heterogeneity in one trial as observed by a metaregression.
  
  
  CONCLUSIONS
  Most of the sample of recent meta-analyses showed that there were signs of imbalance and/or heterogeneity in ages between treatment groups, when there should have been none. Systematic reviewers might consider using the techniques described here to assess the validity of their findings.
authors.affiliation: Department of Health Sciences, York Trials Unit, University of York, York YO10 5DD, United Kingdom. Electronic address: Laura.clark@york.ac.uk.
authors.forename: Laura
authors.initials: L
authors.lastname: Clark
authors.affiliation: Department of Health Sciences, York Trials Unit, University of York, York YO10 5DD, United Kingdom.
authors.forename: Caroline
authors.initials: C
authors.lastname: Fairhurst
authors.affiliation: Department of Health Sciences, York Trials Unit, University of York, York YO10 5DD, United Kingdom.
authors.forename: Catherine E
authors.initials: CE
authors.lastname: Hewitt
authors.affiliation: Department of Social Policy, University of York, York YO10 5DD, United Kingdom.
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Birks
authors.affiliation: Department of Health Sciences, Mental Health and Addictions Research Group, University of York, York YO10 5DD, United Kingdom.
authors.forename: Sally
authors.initials: S
authors.lastname: Brabyn
authors.affiliation: Department of Health Sciences, York Trials Unit, University of York, York YO10 5DD, United Kingdom.
authors.forename: Sarah
authors.initials: S
authors.lastname: Cockayne
authors.affiliation: Department of Health Sciences, York Trials Unit, University of York, York YO10 5DD, United Kingdom.
authors.forename: Sara
authors.initials: S
authors.lastname: Rodgers
authors.affiliation: Department of Health Sciences, York Trials Unit, University of York, York YO10 5DD, United Kingdom.
authors.forename: Katherine
authors.initials: K
authors.lastname: Hicks
authors.affiliation: Department of Health Sciences, York Trials Unit, University of York, York YO10 5DD, United Kingdom.
authors.forename: Robert
authors.initials: R
authors.lastname: Hodgson
authors.affiliation: Department of Health Sciences, Mental Health and Addictions Research Group, University of York, York YO10 5DD, United Kingdom.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Littlewood
authors.affiliation: Department of Health Sciences, York Trials Unit, University of York, York YO10 5DD, United Kingdom.
authors.forename: David J
authors.initials: DJ
authors.lastname: Torgerson
mesh_terms: D015982:Bias; D006801:Humans; D016013:Likelihood Functions; D015201:Meta-Analysis as Topic; D016032:Randomized Controlled Trials as Topic / Q000379:methods*; D012196:Review Literature as Topic
keywords: Heterogeneity; Meta-analysis; Methods; Randomized controlled trials; Selection bias; Systematic review