期刊
STATISTICS IN MEDICINE
卷 37, 期 30, 页码 4652-4664出版社
WILEY
DOI: 10.1002/sim.7943
关键词
cluster randomized trial; design effect; stepped wedge; unequal cluster sizes
类别
资金
- National Institute for Health Research (NIHR) Collaborations for Leadership in Applied Health Research and Care for West Midlands (CLAHRC WM)
- HiSLAC study (NIHR Health Services and Delivery Research Programme) [12/128/07]
Background: A cluster trial with unequal cluster sizes often has lower precision than one with equal clusters, with a corresponding inflation of the design effect. For parallel group trials, adjustments to the design effect are available under sampling models with a single intracluster correlation. Design effects for equal clusters under more complex scenarios have appeared recently (including stepped wedge trials under cross-sectional or longitudinal sampling). We investigate the impact of unequal cluster size in these more general settings. Results: Assuming a linear mixed model with an exchangeable correlation structure that incorporates cluster and subject autocorrelation, we compute the relative efficiency (RE) of a trial with clusters of unequal size under a size-stratified randomization scheme, as compared to an equal cluster trial with the same total number of observations. If there are no within-cluster time effects, the RE exceeds that for a parallel trial. In general, the RE is a weighted average of the RE for a parallel trial and the RE for a crossover trial in the same clusters. Existing approximations for parallel designs are extended to the general setting. Increasing the cluster size by the factor (1 + CV2), where CV is the coefficient of variation of cluster size, leads to conservative sample sizes, as in a popular method for parallel trials. Conclusion: Methods to assess experimental precision for single-period parallel trials with unequal cluster sizes can be extended to stepped wedge and other complete layouts under longitudinal or cross-sectional sampling. In practice, the loss of precision due to unequal cluster sizes is unlikely to exceed 12%.
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