title: Overstating the evidence: double counting in meta-analysis and related problems.
abstract: BACKGROUND
  The problem of missing studies in meta-analysis has received much attention. Less attention has been paid to the more serious problem of double counting of evidence.
  
  
  METHODS
  Various problems in overstating the precision of results from meta-analyses are described and illustrated with examples, including papers from leading medical journals. These problems include, but are not limited to, simple double counting of the same studies, double counting of some aspects of the studies, inappropriate imputation of results, and assigning spurious precision to individual studies.
  
  
  RESULTS
  Some suggestions are made as to how the quality and reliability of meta-analysis can be improved. It is proposed that the key to quality in meta-analysis lies in the results being transparent and checkable.
  
  
  CONCLUSIONS
  Existing quality check lists for meta-analysis do little to encourage an appropriate attitude to combining evidence and to statistical analysis. Journals and other relevant organisations should encourage authors to make data available and make methods explicit. They should also act promptly to withdraw meta-analyses when mistakes are found.
authors.affiliation: Department of Statistics, University of Glasgow, Glasgow, UK. stephen@stats.gla.ac.uk
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Senn
mesh_terms: D002986:Clinical Trials as Topic; D003627:Data Interpretation, Statistical; D006801:Humans; D015201:Meta-Analysis as Topic; D008722:Methods; D016032:Randomized Controlled Trials as Topic; D012196:Review Literature as Topic