title: A review of the use of covariates in cluster randomized trials uncovers marked discrepancies between guidance and practice.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  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.
authors: Centre for Primary Care and Public Health, Blizard Institute, Queen Mary University of London, Yvonne Carter Building, 58 Turner Street, London, E1 2AB, United Kingdom. Electronic address: n.wright@qmul.ac.uk.
authors: Neil
authors: N
authors: Wright
authors: Family Practice Health Centre and Institute for Health Systems Solutions and Virtual Care, Women's College Hospital, 76 Grenville Street, Toronto, ON M5S1B2, Canada; Department of Family and Community Medicine, University of Toronto, 500 University Avenue, 5th Floor, Toronto, ON M5G1V7, Canada.
authors: Noah
authors: N
authors: Ivers
authors: Centre for Primary Care and Public Health, Blizard Institute, Queen Mary University of London, Yvonne Carter Building, 58 Turner Street, London, E1 2AB, United Kingdom.
authors: Sandra
authors: S
authors: Eldridge
authors: Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa Hospital, Civic Campus, 1053 Carling Avenue, Civic Box 693, Ottawa, Ontario K1Y 4E9, Canada; Department of Epidemiology and Community Medicine, University of Ottawa, Ottawa, Ontario, Canada.
authors: Monica
authors: M
authors: Taljaard
authors: Centre for Primary Care and Public Health, Blizard Institute, Queen Mary University of London, Yvonne Carter Building, 58 Turner Street, London, E1 2AB, United Kingdom.
authors: Stephen
authors: S
authors: Bremner
mesh_terms: D016000:Cluster Analysis; D019983:Guideline Adherence / Q000706:statistics & numerical data*; D017410:Practice Guidelines as Topic; D011897:Random Allocation; D016032:Randomized Controlled Trials as Topic / Q000379:methods* / Q000592:standards*; D012107:Research Design / Q000592:standards; D058028:Research Report
keywords: Adjusted analyses; Baseline imbalance; CONSORT statement; Cluster randomized trials; Covariates; Restricted randomization
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