What is the purpose of randomization concealment?

• A. To speed up enrollment
• B. To allow researchers to choose the treatment
• C. To reveal allocation after assignment
• D. To prevent selection bias by keeping allocation unknown until assignment

Answer: (D)

Allocation concealment is about hiding the upcoming treatment assignment from the person enrolling participants, so the assignment remains unknown until the moment the participant is actually assigned. This prevents selection bias, because if the allocator can predict or influence which treatment comes next, they might steer enrolling patients with certain characteristics into one group, skewing the study’s balance. By using mechanisms like centralized randomization, sealed opaque envelopes, or secure online systems that reveal the assignment only after enrollment, the randomization process stays unbiased and the groups stay comparable at baseline. This concept is different from blinding, which aims to prevent bias after assignment by keeping participants or assessors unaware of the treatment. The goal isn’t to speed enrollment or to let researchers choose the treatment, and it isn’t about revealing allocation before or during enrollment—it's about keeping the allocation unknown until the point of assignment to safeguard trial integrity.