Allocation Concealment

Allocation concealment refers to the stringent precautions and methodological steps taken to ensure that the group assignment of patients is not revealed prior to definitively allocating them to their respective groups.
In the hierarchy of establishing a randomized controlled trial (RCT), randomizing patients involves two distinct processes: generating an unpredictable sequence of allocations, and subsequently enforcing allocation concealment.
The practice essentially involves hiding the allocation schedule from the caregivers and researchers who are actively recruiting and enrolling patients into the trial.

The primary purpose of allocation concealment is to prevent caregivers or investigators from working out or predicting in advance the specific allocation of the next eligible participant.
Lack of advance knowledge is vital because it prevents researchers from preferentially allocating specific study participants to a desired trial arm, which would violate the principles of randomization.
For instance, if the allocation schedule is not concealed, a well-intentioned researcher might consciously or subconsciously recruit unhealthier or more severe patients specifically into the active intervention arm, acting on the unproven belief that the participant is likely to benefit more from the new treatment.
By strictly concealing the allocation sequence, the trial minimizes the chance of selection bias and ensures that the baseline characteristics of the treatment and control groups remain balanced and comparable.

While both concepts deal with hiding information to prevent bias, they occur at different stages of a clinical trial.
Allocation concealment is implemented before and during the time a patient is recruited and assigned to a group, ensuring an unbiased enrollment process.
Blinding (or masking), on the other hand, is implemented after the patient has been definitively allocated to a group, preventing the patient or doctor from knowing which specific treatment is actively being administered.