Make your assessment: Allocation Concealment

How did you do? For reference, here’s a chart of the allocation concealment methods that have a “low risk of bias” and those with a “high risk of bias.”

Low risk of bias

High risk of bias

Central allocation Open allocation
Sequentially numbered, opaque, sealed envelopes Envelopes that are translucent, unsealed, or labeled with a numerical pattern.
Sealed pill bottles (or identical looking pills) labeled in numerical order Allocation that allows breaking the randomization sequence, such as surgeon preference post-randomization

When you’re assessing a study’s allocation concealment method (for possible selection bias), you should check to see if its allocation concealment method falls in the “low risk of bias” or “high risk of bias” column. If the method isn’t described, or if you’re unsure, you should label this domain as “unclear.”

So far, we’ve talked about domains that describe the group selection process and help you assess possible selection bias. Let’s move on to the domains that address what happens after participants are assigned to study groups.