Allocation Concealment: Definition and Examples

RCTs > Allocation Concealment

What is Allocation Concealment?

allocation concealmentAllocation concealment is a when the person (or system) responsible for allocating people to different treatment groups and control groups:

  • Doesn’t know what the next treatment allocation will be and
  • Conceals the results of the allocation from others.

Allocation concealment is different from blinding. With blinding, the study personnel and/or the patient does not know which group they have been assigned to. With concealment, the study personnel and the patients have no way of finding out which group (treatment or control) they have been assigned to. While double blinding can reduce bias, allocation concealment is far more powerful.

Without concealment example: An investigator uses a random process to assign patients to treatment groups and control groups. The list is emailed to the study personnel, which essentially means anyone has access to the list. Study personnel could route friends, family, or people with good prognoses to the treatment group and vice versa. This is not an oddity: study personnel have been known to go to extraordinary lengths to reveal patients’ allocations and re-allocate them to a more desirable group.

With concealment example: A Centralized telephone randomization center (like this one) uses randomization to allocate patients to groups. The database is independent and thus is not accessible to study personnel except to receive a treatment assignment for a specific patient.

Random allocation is essential for randomized controlled trials and is considered one of the hallmarks of a quality trial. Allocation concealment is a way to ensure proper randomization; without it, selection bias and confounding biases render the study invalid. In addition, trials that don’t have proper concealment may report much larger treatment effects (as high as 40% more!).


Berger, V. (2020). Randomization, Masking, and Allocation Concealment. Chapman & Hall/CRC.
Pidal J, Hrobjartsson A, et al. Impact of Alloc. concealment on conclusions drawn from meta-analyses of randomized trials. Int J Epidemiol 2007;36:847-857.
Schulz KF, Grimes DA. Alloc. concealment in randomized trials: defending against deciphering. The Lancet 2002;359(9306):614-18.

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