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 = 7162) and parallel arm (n = 132, sample size n = 11 398) trials. Effect sizes correlated well with the two types of designs (ρ0.72). 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. © The Author 2007; all rights reserved.