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OBJECTIVES: Reviews of the handling of covariates in trials have explicitly excluded cluster randomized trials (CRTs). In this study, we review the use of covariates in randomization, the reporting of covariates, and adjusted analyses in CRTs. STUDY DESIGN AND SETTING: We reviewed a random sample of 300 CRTs published between 2000 and 2008 across 150 English language journals. RESULTS: Fifty-eight percent of trials used covariates in randomization. Only 69 (23%) included tables of cluster- and individual-level covariates. Fifty-eight percent reported significance tests of baseline balance. Of 207 trials that reported baseline measures of the primary outcome, 155 (75%) subsequently adjusted for these in analyses. Of 174 trials that used covariates in randomization, 30 (17%) included an analysis adjusting for all those covariates. Of 219 trial reports that included an adjusted analysis of the primary outcome, only 71 (32%) reported that covariates were chosen a priori. CONCLUSION: There are some marked discrepancies between practice and guidance on the use of covariates in the design, analysis, and reporting of CRTs. It is essential that researchers follow guidelines on the use and reporting of covariates in CRTs, promoting the validity of trial conclusions and quality of trial reports.

Original publication




Journal article


J Clin Epidemiol

Publication Date





603 - 609


Adjusted analyses, Baseline imbalance, CONSORT statement, Cluster randomized trials, Covariates, Restricted randomization, Cluster Analysis, Guideline Adherence, Practice Guidelines as Topic, Random Allocation, Randomized Controlled Trials as Topic, Research Design, Research Report