title: Evidence from crossover trials: empirical evaluation and comparison against parallel arm trials.
abstract: BACKGROUND
  We aimed to evaluate empirically how crossover trial results are analysed in meta-analyses of randomized evidence and whether their results agree with parallel arm studies on the same questions.
  
  
  METHODS
  We used a systematic sample of Cochrane meta-analyses including crossover trials. We evaluated the methods of analysis for crossover results and compared the concordance of the estimated effect sizes in crossover vs parallel arm trials.
  
  
  RESULTS
  Of 334 screened reviews, 62 had crossover trials. Of those, 33 meta-analyses performed quantitative syntheses involving two-arm two-period crossover trials. There was large variability on how these trials were analysed; only one of the 33 meta-analyses stated that they used the data from both the first and second period with an appropriate paired approach. Nine meta-analyses used the first period data only and 14 gave no information at all on what they had done. Twenty-eight meta-analyses had both crossover (n = 137, sample size n = 7,162) and parallel arm (n = 132, sample size n = 11,398) trials. Effect sizes correlated well with the two types of designs (rho = 0.72). Differences on whether the summary effect had a P < 0.05 or not were common due to limited sample sizes. The summary relative odds ratio for parallel arm vs crossover designs for favourable outcomes was 0.87 (95% CI, 0.74-1.02).
  
  
  CONCLUSIONS
  Crossover designs may contribute evidence in a fifth of systematic reviews, but few meta-analyses make use of their full data. The results of crossover trials tend to agree with those of parallel arm trials, although there was a trend for more conservative treatment effect estimates in parallel arm trials.
authors.affiliation: Clinical Trials and Evidence-Based Medicine Unit, Department of Hygiene and Epidemiology, University of Ioannina School of Medicine, Ioannina, Greece.
authors.forename: Dimitrios N
authors.initials: DN
authors.lastname: Lathyris
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Trikalinos
authors.forename: John P  A
authors.initials: JP
authors.lastname: Ioannidis
mesh_terms: D018592:Cross-Over Studies; D005069:Evaluation Studies as Topic; D005260:Female; D006801:Humans; D008297:Male; D015201:Meta-Analysis as Topic; D016032:Randomized Controlled Trials as Topic / Q000592:standards*