Stratification is a method used in randomized clinical trials to balance prognostic factors between study groups as simple randomization does not remove all imbalances.

The study cohort is separated into strata according to the chosen prognostic factor and then randomized. This allows researchers to take factors such as age and gender, which may affect outcome into account.

Source: Lim, C. Y., & In, J. (2019). Randomization in clinical studies. Korean journal of anesthesiology, 72(3), 221–232.