Intended for healthcare professionals


Internal and external validity of cluster randomised trials: systematic review of recent trials

BMJ 2008; 336 doi: (Published 17 April 2008) Cite this as: BMJ 2008;336:876
  1. Sandra Eldridge, professor of biostatistics1,
  2. Deborah Ashby, professor of medical statistics2,
  3. Catherine Bennett, statistician1,
  4. Melanie Wakelin, lecturer in medical statistics1,
  5. Gene Feder, professor of primary care research and development1
  1. 1Centre for Health Sciences, Barts and The London School of Medicine and Dentistry, London E1 2AT
  2. 2Wolfson Institute of Preventive Medicine, Barts and The London School of Medicine and Dentistry, London EC1M 6BQ
  1. Correspondence to: S Eldridge s.eldridge{at}
  • Accepted 25 February 2008


Objectives To assess aspects of the internal validity of recently published cluster randomised trials and explore the reporting of information useful in assessing the external validity of these trials.

Design Review of 34 cluster randomised trials in primary care published in 2004 and 2005 in seven journals (British Medical Journal, British Journal of General Practice, Family Practice, Preventive Medicine, Annals of Internal Medicine, Journal of General Internal Medicine, Pediatrics).

Data sources National Library of Medicine (Medline) via PubMed.

Data extraction To assess aspects of internal validity we extracted data on appropriateness of sample size calculations and analyses, methods of identifying and recruiting individual participants, and blinding. To explore reporting of information useful in assessing external validity we extracted data on cluster eligibility, cluster inclusion and retention, cluster generalisability, and the feasibility and acceptability of the intervention to health providers in clusters.

Results 21 (62%) trials accounted for clustering in sample size calculations and 30 (88%) in the analysis; about a quarter were potentially biased because of procedures surrounding recruitment and identification of patients; individual participants were blind to allocation status in 19 (56%) and outcome assessors were blind in 15 (44%). In almost half the reports, information relating to generalisability of clusters was poorly reported, and in two fifths there was no information about the feasibility and acceptability of the intervention.

Conclusions Cluster randomised trials are essential for evaluating certain types of interventions. Issues affecting their internal validity, such as appropriate sample size calculations and analysis, have been widely disseminated and are now better addressed by researchers. Blinding of those identifying and recruiting patients to allocation status is recommended but is not always carried out. There may be fewer barriers to internal validity in trials in which individual participants are not recruited. External validity seems poorly addressed in many trials, yet is arguably as important as internal validity in judging quality as a basis for healthcare intervention.


  • Contributions: SE conceived the idea for the study, led the research, and wrote the initial draft. GF and DA contributed to design and interpretation. MW and CB extracted data. All authors contributed to the final paper. SE is guarantor.

  • Funding: SE received a HEFC promising research fellowship.

  • Competing interests: None declared.

  • Ethics approval: Not required.

  • Provenance and peer review: Not commissioned; externally peer reviewed.

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