Back to: Assess for Risk of Bias

Let’s say a study used a computer algorithm to randomly assign participants into groups. The groups ought to be similar assuming the randomization was done correctly, right?

Unfortunately, there’s still an opportunity for selection bias.

Sometimes, the people who assign participants into groups could, either consciously or subconsciously, manipulate the group assignment process.

Allocation concealment is the method studies use to prevent this from happening. It describes how researchers hide the sequence for group assignments from the person who is doing the assigning, so that this person can’t predict what’s coming next.

Protects against:  Selection bias

If a researcher knows what group the next participant will be assigned to before the participant walks in the door, the researcher could manipulate the process and create uneven groups

For instance, if the study manager knows that the next participant who walks in the door will be assigned to the treatment group, she might let a sick patient who needs that treatment cut to the front of the line.

Examples of Allocation concealment:

Allocation concealment can be as simple as using opaque, sealed envelopes and as complicated as having a pharmacy hold the random sequence and only give the information to researchers one participant at a time.

How to assess this domain:

First, look for evidence that study researchers used a technique to hide the sequence for group assignments from the individual who was doing the assigning. Then, decide if this technique was likely to be effective in preventing selection bias.