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JAMA Guide to Statistics and Methods
June 5, 2018

Randomization in Clinical Trials: Permuted Blocks and Stratification

Author Affiliations
  • 1Berry Consultants LLC, Austin, Texas
JAMA. 2018;319(21):2223-2224. doi:10.1001/jama.2018.6360

The most compelling way to establish that an intervention definitively causes a clinical outcome is to randomly allocate patients into treatment groups. Randomization helps to ensure that a certain proportion of patients receive each treatment and that the treatment groups being compared are similar in both measured and unmeasured patient characteristics.1,2 Simple or unrestricted, equal randomization of patients between 2 treatment groups is equivalent to tossing a fair coin for each patient assignment.2,3 As the sample size increases, the 2 groups will become more perfectly balanced. However, this balance is not guaranteed when there are relatively few patients enrolled in a trial. In the coin toss scenario, obtaining several consecutive heads, for example, is more likely than typically perceived.1,4 If a long series of assignments to 1 group occurred when randomizing patients in a clinical trial, imbalances between the groups would occur.

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