During journal clubs on randomized controlled trials there is often confusion about allocation concealment. It is often confused with blinding. In a sense it is blinding but not in the traditional sense of blinding. One way to think of allocation concealment is blinding of the randomization schedule or scheme. Allocation concealment hides the randomization or allocation sequence (what’s coming next) from patients and those who would enroll patients in a study. Blinding occurs after randomization and keeps patients, providers, researchers, etc from knowing which arm of the study the patient is in (i.e. what treatment they are getting).
Why is allocation concealment important in a randomized controlled trial? Inadequate or unclear allocation concealment can lead to an overestimation (by up to 40%!) of treatment effect (JAMA 1995;273:408). First, consider why we randomize in the first place. We randomize to try to equally distribute confounding and prognostic factors between arms of a study so we can try to isolate the effect of the intervention. Consider a physician who wants to enroll a patient in a study and wants to make sure her patient receives the therapy she deems likely most effective. What if she figured out the randomization scheme and knows what therapy the next patient will be assigned to? Hopefully you can see that this physician could undermine the benefits of randomization if she preferentially funnels sicker (or healthier) patients into one arm of the study. There could be an imbalance in baseline characteristics. It could also lead to patients who are enrolled in the study being fundamentally different or not representative of the patient population.
You will have to use your judgment to decide how likely it is that someone could figure out the randomization scheme. You can feel more comfortable that allocation concealment was adequate if the following were used in the RCT:
– sequentially numbered, opaque, sealed envelopes: these are not able to be seen through even if held up to a light. They are sealed so that you can’t peek into them and see what the assignment is. As each patient is enrolled you use the next numbered envelope.
– pharmacy controlled: enrolling physician calls the pharmacy and they enroll the patient and assign therapy.
– centralized randomization: probably the most commonly used. The enrolling physician calls a central research site and the central site assigns the patient to therapy.
Proper randomization is crucial to a therapy study and concealed allocation is crucial to randomization. I hope this post helps readers of RCTs better understand what concealed allocation is and learn how to detect whether it was done adequately or not. Keep in mind if allocation concealment is unclear or done poorly the effect you see in the study needs to be tempered and possible cut by 40%.