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Cited by 224Statistical Methods in Clinical TrialsMeta-analysis and systematic reviewsHealth Policy Implementation Science

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

Open access
Eldridge, Sandra, Ashby, Deborah, Bennett, Catherine, Wakelin, Melanie, Feder, Gene·2008·BMJ
DOI 10.1136/bmj.39517.495764.25PMC PMC2323095
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Abstract

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.

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  • Others have assessed about 40% of cluster trials to be at risk of these types of biases; and sometimes this has been reported to be somewhat lower despite including many trials with post randomization recruitment.

    “Others have assessed about 40% of cluster trials to be at risk of these types of biases [2,7,28]; and sometimes this has been reported to be somewhat lower despite including many trials with post randomization recruitment [10,13].”

    From Cluster randomized trials of individual-level interventions were at high risk of bias (2021) · cites this paper as [10]

  • Additional elements for trial designs other than parallel group superiority trials are often not described in final trial reports and are infrequently specified in the protocol.

    “Such elements are often not described in final trial reports,299 300 301 302 and they are infrequently specified in the protocol.303”

    From SPIRIT 2025 explanation and elaboration: updated guideline for protocols of randomised trials (2025) · cites this paper as [300]

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Research works printed but not linked: 2

  1. Murray DM. Design and analysis of group randomised trials. New York: Oxford University Press, 1998.
  2. Kerry SM, Bland JM. Sample size in cluster randomisation. BMJ 1998;316:549.

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Authors
Eldridge, S., Ashby, D., Bennett, C., Wakelin, M., & Feder, G.
Affiliations
Queen Mary University of London (GB)
Corresponding author
Eldridge, Sandra
ORCID
Venue
BMJ
Publisher
BMJ
Published
2008-03-25
Volume
336
Issue
7649
Pages
876-880
ISSN
  • 1756-1833 (ISSN-L)
  • 0959-8138
DOI
10.1136/bmj.39517.495764.25
PMC
PMC2323095
OpenAlex
W2047660576
Cited by
224
References
41
Citations by year
2026: 6 · 2025: 14 · 2024: 8 · 2023: 12 · 2022: 5 · 2021: 15 · 2020: 6 · 2019: 8 · 2018: 11 · 2017: 11 · 2016: 16 · 2015: 7 · 2014: 9 · 2013: 20 · 2012: 23
FWCI
2.55
Type
journal article
Language
en
Open access
Licence
www.bmj.org/licenses/tdm/1.0/terms-and-conditions.html (tdm)
Locations
  • BMJ (publishedVersion)
  • PubMed Central (publishedVersion)
  • PubMed (publishedVersion)
  • HighWire Press Open Archive (submittedVersion)
  • Bristol Research (University of Bristol) (submittedVersion)
  • Explore Bristol Research (submittedVersion)
Topics
  • Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Clinical Trials
  • Social Sciences › Decision Sciences › Statistics, Probability and Uncertainty › Meta-analysis and systematic reviews
  • Health Sciences › Health Professions › General Health Professions › Health Policy Implementation Science
Keywords
internal validity, generalizability, trial reporting quality, sample size determination, blinding, participant recruitment, feasibility and acceptability
MeSH terms
Reproducibility of Results, Cluster Analysis, Randomized Controlled Trials as Topic, Sample Size

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