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Risk of selection bias in randomised trials
Open accessAbstract
BACKGROUND: Selection bias occurs when recruiters selectively enrol patients into the trial based on what the next treatment allocation is likely to be. This can occur even if appropriate allocation concealment is used if recruiters can guess the next treatment assignment with some degree of accuracy. This typically occurs in unblinded trials when restricted randomisation is implemented to force the number of patients in each arm or within each centre to be the same. Several methods to reduce the risk of selection bias have been suggested; however, it is unclear how often these techniques are used in practice. METHODS: We performed a review of published trials which were not blinded to assess whether they utilised methods for reducing the risk of selection bias. We assessed the following techniques: (a) blinding of recruiters; (b) use of simple randomisation; (c) avoidance of stratification by site when restricted randomisation is used; (d) avoidance of permuted blocks if stratification by site is used; and (e) incorporation of prognostic covariates into the randomisation procedure when restricted randomisation is used. We included parallel group, individually randomised phase III trials published in four general medical journals (BMJ, Journal of the American Medical Association, The Lancet, and New England Journal of Medicine) in 2010. RESULTS: We identified 152 eligible trials. Most trials (98%) provided no information on whether recruiters were blind to previous treatment allocations. Only 3% of trials used simple randomisation; 63% used some form of restricted randomisation, and 35% did not state the method of randomisation. Overall, 44% of trials were stratified by site of recruitment; 27% were not, and 29% did not report this information. Most trials that did stratify by site of recruitment used permuted blocks (58%), and only 15% reported using random block sizes. Many trials that used restricted randomisation also included prognostic covariates in the randomisation procedure (56%). CONCLUSIONS: The risk of selection bias could not be ascertained for most trials due to poor reporting. Many trials which did provide details on the randomisation procedure were at risk of selection bias due to a poorly chosen randomisation methods. Techniques to reduce the risk of selection bias should be more widely implemented.
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“This finding is consistent with findings in individual randomized trials which have also been identified at risk of bias due to implementation of the randomization process [21].”
From Cluster randomized trials of individual-level interventions were at high risk of bias (2021) · cites this paper as [21]
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Metadata
- Authors
- Kahan, B. C., Rehal, S., & Cro, S.
- Affiliations
- Queen Mary University of London (GB)
- MRC Clinical Trials Unit at UCL (GB)
- Corresponding author
- Kahan, Brennan C.
- Venue
- Trials
- Published
- 2015-09-10
- Volume
- 16
- Issue
- 1
- Pages
- 405-405
- ISSN
- 1745-6215 (ISSN-L)
- PMC
- PMC4566301
- OpenAlex
- W1703551835
- Cited by
- 180
- References
- 33
- Citations by year
- 2026: 12 · 2025: 21 · 2024: 21 · 2023: 18 · 2022: 16 · 2021: 19 · 2020: 36 · 2019: 11 · 2018: 10 · 2017: 11 · 2016: 3 · 2015: 1
- FWCI
- 7.42
- Type
- journal article
- Language
- en
- Open access
- gold
- doi.org
- Licence
- Locations
- Trials (publishedVersion, cc-by)
- Europe PMC (PubMed Central) (publishedVersion, cc-by)
- PubMed Central (publishedVersion, cc-by)
- PubMed (publishedVersion)
- UCL Discovery (University College London) (submittedVersion)
- Queen Mary Research Online (Queen Mary University of London) (submittedVersion, cc-by)
- Spiral (Imperial College London) (submittedVersion, cc-by)
- Funders
- Medical Research Council (MC_UU_12023/29)
- 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
- Physical Sciences › Mathematics › Statistics and Probability › Advanced Causal Inference Techniques
- Keywords
- selection bias, randomization methods, allocation concealment, prognostic factors, reporting quality
- MeSH terms
- Humans, Selection Bias, Randomized Controlled Trials as Topic, Clinical Trials, Phase III as Topic, Sample Size, Patient Selection
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