Allocation concealment in randomised trials: defending against deciphering

Schulz, Kenneth F; Grimes, David A

doi:10.1016/s0140-6736(02)07750-4

Cited by 923Statistical Methods in Clinical TrialsMeta-analysis and systematic reviewsAdvanced Causal Inference Techniques

Allocation concealment in randomised trials: defending against deciphering

Schulz, Kenneth F, Grimes, David A·2002·The Lancet

DOI 10.1016/s0140-6736(02)07750-4

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Abstract

Proper randomisation rests on adequate allocation concealment. An allocation concealment process keeps clinicians and participants unaware of upcoming assignments. Without it, even properly developed random allocation sequences can be subverted. Within this concealment process, the crucial unbiased nature of randomised controlled trials collides with their most vexing implementation problems. Proper allocation concealment frequently frustrates clinical inclinations, which annoys those who do the trials. Randomised controlled trials are anathema to clinicians. Many involved with trials will be tempted to decipher assignments, which subverts randomisation. For some implementing a trial, deciphering the allocation scheme might frequently become too great an intellectual challenge to resist. Whether their motives indicate innocent or pernicious intents, such tampering undermines the validity of a trial. Indeed, inadequate allocation concealment leads to exaggerated estimates of treatment effect, on average, but with scope for bias in either direction. Trial investigators will be crafty in any potential efforts to decipher the allocation sequence, so trial designers must be just as clever in their design efforts to prevent deciphering. Investigators must effectively immunise trials against selection and confounding biases with proper allocation concealment. Furthermore, investigators should report baseline comparisons on important prognostic variables. Hypothesis tests of baseline characteristics, however, are superfluous and could be harmful if they lead investigators to suppress reporting any baseline imbalances.

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Using a validated method of randomisation helps minimise selection bias and reduce systematic differences in the characteristics of animals allocated to different groups.
“Using a validated method of randomisation helps minimise selection bias and reduce systematic differences in the characteristics of animals allocated to different groups [62–64].”
From Reporting animal research: Explanation and elaboration for the ARRIVE guidelines 2.0 (2020) · cites this paper as [63]
Successful randomisation in practice depends on two interrelated factors: generation of an unpredictable allocation sequence and concealment of that sequence until assignment irreversibly occurs.
“Successful randomisation in practice depends on two interrelated factors: generation of an unpredictable allocation sequence (item 21), and concealment of that sequence until assignment irreversibly occurs.336 346”
From SPIRIT 2025 explanation and elaboration: updated guideline for protocols of randomised trials (2025) · cites this paper as [346]
Without adequate allocation concealment, even random, unpredictable assignment sequences can be subverted.
“Without adequate allocation concealment, even random, unpredictable assignment sequences can be subverted.336 346 347 348”
From SPIRIT 2025 explanation and elaboration: updated guideline for protocols of randomised trials (2025) · cites this paper as [346]
The allocation concealment mechanism aims to prevent participants and recruiters from knowing the study group to which the next participant will be assigned.
“The allocation concealment mechanism aims to prevent participants and recruiters from knowing the study group to which the next participant will be assigned.336 346”
From SPIRIT 2025 explanation and elaboration: updated guideline for protocols of randomised trials (2025) · cites this paper as [346]

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Authors: Schulz, K. F., & Grimes, D. A.
Affiliations: Family Health International 360 (US)
Corresponding author: Schulz, Kenneth F
ORCID: Schulz, Kenneth F
Grimes, David A
Venue: The Lancet
Publisher: Elsevier BV
Published: 2002-02
Volume: 359
Issue: 9306
Pages: 614-618
ISSN: 0099-5355 (ISSN-L)
0140-6736
DOI: 10.1016/s0140-6736(02)07750-4
OpenAlex: W1976357112
Cited by: 923
References: 37
Citations by year: 2026: 51 · 2025: 29 · 2024: 33 · 2023: 41 · 2022: 34 · 2021: 32 · 2020: 41 · 2019: 33 · 2018: 39 · 2017: 35 · 2016: 59 · 2015: 46 · 2014: 38 · 2013: 52 · 2012: 49
FWCI: 5.05
Type: journal article
Language: en
Open access: closed
Licence: www.elsevier.com/tdm/userlicense/1.0/ (tdm)
Locations: The Lancet (publishedVersion)
PubMed (publishedVersion)
DigitalGeorgetown (Georgetown University Library) (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
Physical Sciences › Mathematics › Statistics and Probability › Advanced Causal Inference Techniques
Keywords: allocation concealment, randomized controlled trials, selection bias, confounding, baseline imbalance, prognostic factors, trial design, treatment effect estimation
MeSH terms: Adult, Humans, Random Allocation, Reproducibility of Results, Confounding Factors, Epidemiologic, Randomized Controlled Trials as Topic

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Allocation concealment in randomised trials: defending against deciphering

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