title: Important outcome predictors showed greater baseline heterogeneity than age in two systematic reviews.
abstract: OBJECTIVE
  An unknown number of randomized controlled trials (RCTs) have their treatment allocation subverted. If such trials are included in systematic reviews, biased results may be used to change policy. To assess whether a systematic review contains subverted trials, a meta-analysis of group differences regarding a baseline variable can be undertaken. In this article, the performance of age with another prognostic variable in detecting selection bias within systematic reviews is compared.
  
  
  METHODS
  Two Cochrane systematic reviews, one of low back pain and one of hip protectors for fracture prevention, were identified. The component RCT texts were obtained, and data were extracted on age, baseline back pain score (low back pain review), and baseline body mass (hip protector review). In this exemplar, we tested for baseline heterogeneity with a fixed-effects meta-analysis.
  
  
  RESULTS
  Heterogeneity in age between the intervention and control groups was found. The observed heterogeneity increased with baseline back pain and body mass relative to age in each review.
  
  
  CONCLUSIONS
  We found that covariates predictive of outcome demonstrate greater heterogeneity than age. However, there were fewer missing data relating to age. Reviewers should consider using age and another prognostic covariate in baseline meta-analyses to check the validity of their results.
authors.affiliation: York Trials Unit, Department of Health Sciences, University of York, Heslington, York, YO10 5DD, North Yorkshire, UK. Electronic address: laura.clark@york.ac.uk.
authors.forename: Laura
authors.initials: L
authors.lastname: Clark
authors.affiliation: York Trials Unit, Department of Health Sciences, University of York, Heslington, York, YO10 5DD, North Yorkshire, UK.
authors.forename: Caroline
authors.initials: C
authors.lastname: Fairhurst
authors.affiliation: York Trials Unit, Department of Health Sciences, University of York, Heslington, York, YO10 5DD, North Yorkshire, UK.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Cook
authors.affiliation: York Trials Unit, Department of Health Sciences, University of York, Heslington, York, YO10 5DD, North Yorkshire, UK.
authors.forename: David J
authors.initials: DJ
authors.lastname: Torgerson
mesh_terms: D000328:Adult; D000367:Age Factors; D000369:Aged, 80 and over; D015982:Bias; D016022:Case-Control Studies; D050723:Fractures, Bone / Q000628:therapy*; D006801:Humans; D016013:Likelihood Functions; D017116:Low Back Pain / Q000628:therapy*; D015201:Meta-Analysis as Topic; D017063:Outcome Assessment, Health Care; D011379:Prognosis; D016032:Randomized Controlled Trials as Topic; D012107:Research Design; D012196:Review Literature as Topic; D015983:Selection Bias
keywords: Bias; Heterogeneity; Meta-analysis; Meta-regression; RCTs; Systematic review