Cluster randomized trials produced similar results to individually randomized trials in a meta-analysis of enhanced care for depression

Simon Gilbody, Peter Bower, David Torgerson, David Richards

Research output: Contribution to journalArticlepeer-review


Objectives: To examine whether cluster randomized trials (1) produce baseline imbalances between intervention and control conditions; (2) give results that are substantially different individually randomized trials; and (3) give different results when adjusted for unit of analysis error.

Study Design and Setting: We used 14 cluster randomized trials and 20 individualized trials of the same intervention (collaborative care for depression). We conducted a random effects meta-analysis to examine imbalance in baseline depression scores. We used meta-regression to test for differential effect size and heterogeneity between clustered and individualized studies. Unit of analysis error was corrected using a range of plausible published intraclass correlation coefficients (ICCs).

Results: There were no baseline imbalances in either cluster randomized (P = 0.837) or individually randomized (P = 0.737) studies. Cluster randomized studies gave almost identical estimates of effect size when compared to individually randomized studies (standardized mean difference, SMDcluster = 0.25, 95% confidence interval [CI]: 0.17, 0.33; SMDindividual = 0.24; 95% CI: 0.13, 0.36). Adjustment for clustering had minimal effect on clinical and statistical significance (pooled SMDICC 0.02 = 0.249 [95% CI: 0.174,0.325] to SMDICC (0.05) = 0.258 [95% CI: 0.172, 0.345]).

Conclusion: The additional effort and expense involved in cluster randomized trials needs to be justified when individualized studies might produce robust and believable results. (C) 2008 Elsevier Inc. All rights reserved.

Original languageEnglish
Pages (from-to)160-168
Number of pages9
JournalJournal of Clinical Epidemiology
Issue number2
Publication statusPublished - Feb 2008


  • cluster randomized trials
  • trial design
  • selection bias
  • baseline imbalances
  • meta-analysis
  • depression
  • RISK
  • BIAS

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