Inference for the treatment effect in multiple-period cluster randomised trials when random effect correlation structure is misspecified

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Multiple-period cluster randomised trials, such as stepped wedge or cluster cross-over trials, are being conducted with increasing frequency. In the design and analysis of these trials, it is necessary to specify the form of the within-cluster correlation structure, and a common assumption is that the correlation between the outcomes of any pair of subjects within a cluster is identical. More complex models that allow for correlations within a cluster to decay over time have recently been suggested. However, most software packages cannot fit these models. As a result, practitioners may choose a simpler model. We analytically examine the impact of incorrectly omitting a decay in correlation on the variance of the treatment effect estimator and show that misspecification of the within-cluster correlation structure can lead to incorrect conclusions regarding estimated treatment effects for stepped wedge and cluster crossover trials.

Original languageEnglish
Pages (from-to)3112-3122
Number of pages11
JournalStatistical Methods in Medical Research
Issue number10-11
Publication statusPublished - Nov 2019


  • Analysis of variance
  • cluster randomised trial
  • intracluster correlation
  • model misspecification
  • stepped wedge

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