title: Dealing with substantial heterogeneity in Cochrane reviews. Cross-sectional study.
abstract: BACKGROUND
  Dealing with heterogeneity in meta-analyses is often tricky, and there is only limited advice for authors on what to do. We investigated how authors addressed different degrees of heterogeneity, in particular whether they used a fixed effect model, which assumes that all the included studies are estimating the same true effect, or a random effects model where this is not assumed.
  
  
  METHODS
  We sampled randomly 60 Cochrane reviews from 2008, which presented a result in its first meta-analysis with substantial heterogeneity (I2 greater than 50%, i.e. more than 50% of the variation is due to heterogeneity rather than chance). We extracted information on choice of statistical model, how the authors had handled the heterogeneity, and assessed the methodological quality of the reviews in relation to this.
  
  
  RESULTS
  The distribution of heterogeneity was rather uniform in the whole I2 interval, 50-100%. A fixed effect model was used in 33 reviews (55%), but there was no correlation between I2 and choice of model (P = 0.79). We considered that 20 reviews (33%), 16 of which had used a fixed effect model, had major problems. The most common problems were: use of a fixed effect model and lack of rationale for choice of that model, lack of comment on even severe heterogeneity and of reservations and explanations of its likely causes. The problematic reviews had significantly fewer included trials than other reviews (4.3 vs. 8.0, P = 0.024). The problems became less pronounced with time, as those reviews that were most recently updated more often used a random effects model.
  
  
  CONCLUSIONS
  One-third of Cochrane reviews with substantial heterogeneity had major problems in relation to their handling of heterogeneity. More attention is needed to this issue, as the problems we identified can be essential for the conclusions of the reviews.
authors.affiliation: Nordic Cochrane Centre, Rigshospitalet and University of Copenhagen, Denmark. jeppe@heroes.dk
authors.forename: Jeppe B
authors.initials: JB
authors.lastname: Schroll
authors.forename: Rasmus
authors.initials: R
authors.lastname: Moustgaard
authors.forename: Peter C
authors.initials: PC
authors.lastname: Gøtzsche
mesh_terms: D003430:Cross-Sectional Studies; D003627:Data Interpretation, Statistical; D016208:Databases, Factual; D006801:Humans; D015201:Meta-Analysis as Topic; D015233:Models, Statistical; D012107:Research Design