Allocation concealment — MRCPsych Paper A MCQ

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ModerateEpidemiology and research methodsAllocation concealmentMRCPsych Paper A

In a parallel-group randomised controlled trial, recruiters enter a participant's eligibility and consent details into an independent central web system. Only after these details have been irreversibly recorded does the system reveal the treatment assignment. Recruiters and outcome assessors may know the assigned treatment thereafter. Which methodological safeguard is primarily described?

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Reveal the answer and explanation

Correct answer: D — Allocation concealment

The correct answer is D, allocation concealment. The central web system withholds the forthcoming assignment until eligibility, consent and enrolment are irreversibly established. This prevents recruiters from manipulating enrolment according to the next allocation and therefore limits selection bias. Random sequence generation concerns how an unpredictable sequence is created, whereas allocation concealment concerns preventing foreknowledge while that sequence is implemented. Blinding of outcome assessors occurs after allocation and aims principally to reduce detection bias; the stem explicitly permits assessors to know treatment. Stratified randomisation balances allocation within predefined prognostic subgroups. Cluster randomisation assigns intact groups, such as wards or practices, rather than individual participants.

Reference: Hopewell S, Chan AW, Collins GS, et al. CONSORT 2025 explanation and elaboration: updated guideline for reporting randomised trials, Item 18: mechanism used to implement the random allocation sequence. BMJ. 2025;389:e081124. https://www.bmj.com/content/389/bmj-2024-081124