Question 5 of 9 Advanced Concept
What is the purpose of allocation concealment?
Short answer
Allocation concealment prevents recruiters and investigators from knowing or predicting the next assignment before a participant enters the trial. That reduces selection bias. It is different from blinding, which limits knowledge of treatment assignments after allocation.
Why
Randomization protects internal validity only when allocation is implemented faithfully. If a recruiter can predict the next treatment, they might consciously or unconsciously delay enrollment of particular patients, changing who enters each group. Allocation concealment blocks this opportunity at the recruitment boundary.
For example, a secure central randomization system releases an assignment only after eligibility and consent are confirmed. Properly managed sequentially numbered opaque sealed envelopes can sometimes be used, but they require meticulous safeguards and are easier to compromise. Blinding answers a different question.
It reduces performance, detection or reporting biases after randomization, depending on who is masked. A trial may be open label yet still use excellent allocation concealment. Describe sequence generation, safeguards, assignment implementation and any breaches in the protocol and final report.
Calling a study randomized is not sufficient evidence that allocation was concealed.
Common mistakes
- Equating allocation concealment with double blinding or assuming an unprotected alternating sequence is randomization
The interviewer might ask next
Can an open-label randomized trial still achieve allocation concealment?