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We examine different methods to pool binary outcomes used both in parallel and cross-over trials. Odds ratio (OR) estimators obtained from joint conditional probabilities in cross-over trials, such as the Mantel-Haenszel and Peto methods, are compared to an OR estimator using marginal results of cross-over trials. When there is correlation between the outcomes in the two cross-over periods, joint conditional ORs differ from marginal ORs and cannot be combined with OR estimates from parallel trials. The marginal OR estimate is independent of the between-period correlation and it includes a correction for cross-over correlation in the variance estimate. As its computation is similar in cross-over and parallel trials, it is the method of choice to pool results from parallel and cross-over trials in a combined design meta-analysis.

Original publication




Journal article


Statistics in medicine

Publication Date





2145 - 2159


Medical Statistics Unit, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1 7HT, UK.


Humans, Diabetic Retinopathy, Diabetic Nephropathies, Albuminuria, Glomerular Filtration Rate, Diet, Protein-Restricted, Light Coagulation, Odds Ratio, Cross-Over Studies, Clinical Trials as Topic, Statistics as Topic, Meta-Analysis as Topic