Randomization – Urn

What It Is

Random assignment to study group of later participants is partly based on the assignment received by previously-enrolled participants.  This is done to achieve the intended proportion of participants in each study group (e.g., 50% control and 50% experimental).

Why It’s Important

Simple randomization does not guarantee that the intended proportion of participants assigned to each group will occur, especially in smaller trials (e.g., the trial ends up with 40% control and 60% experimental).  This reduces the statistical power of the trial to identify a difference between control and experimental if that is the true state. It also eliminates a potential problem with block randomization which can reveal the next treatment assignment for a subset of participants.