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Block Randomization with Random Block Lengths

Block randomization suffers from one problem: it can become possible to determine the next group assignment if the block length is short. To combat that, random block lengths can be used. Common lengths are 2, 4, and 6. To ensure balance, the random block lengths must be a multiple of the number of groups.

Note: In order to keep balance, the block length should be divisible by the number of groups.

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  • M. Lachin, John, P. Matts, John & Wei, L.J. (1989), "Randomization in clinical trials: Conclusions and recommendations," Controlled clinical trials, 9, 365-74.
  • Suresh K. (2011), "An overview of randomization techniques: An unbiased assessment of outcome in clinical research," Journal of Human Reproductive Sciences, 4(1), 8-11.
  • Vickers AJ. (2006), "How to randomize," Journal of the Society for Integrative Oncology, 4(4), 194-198.