Which statement best describes allocation concealment in randomized trials?

• A. The process to prevent foreknowledge of treatment assignment, reducing selection bias.
• B. The procedure used to blind participants so they do not know the treatment after assignment.
• C. The method of ensuring equal distribution of prognostic factors by post-randomization adjustments.
• D. The blinding of outcome assessors to treatment group.

Answer: (A)

Allocation concealment means preventing foreknowledge of the treatment assignment as participants are being enrolled. This ensures the person enrolling participants cannot predict or influence which group the next person will join, guarding against selection bias that could distort the comparability of the groups. It is different from blinding, which masks the assigned treatment after allocation to reduce bias during care or outcome assessment. Blinding participants after assignment, or blinding outcome assessors, addresses different biases that arise after the randomization decision, not the leak of information during enrollment. Proper concealment methods include centralized randomization or using opaque, sealed envelopes or a third-party system to reveal the allocation only at the moment of assignment. By keeping the upcoming allocation hidden during enrollment, the study maintains the integrity of randomization and the validity of its comparisons.