Stratified Randomization

What It Is

A separate randomization sequence (allocation, list) is generated for each subgroup of participants to be enrolled in a clinical trial.

Why It’s Important

Since it’s a chance procedure, simple randomization may still produce study groups that are not balanced or equivalent with respect to a key factor that affects the outcome. When this is the case, it may be very difficult to determine why the outcomes of the study groups are different – it might be because of the intervention or because of the underlying risk factor. Stratification of the randomization by an important risk factor ensures that that factor will not be a reason study groups have different outcomes.