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Int J Environ Res Public Health. 2011 Jan;8(1):15-20. doi: 10.3390/ijerph8010015. Epub 2010 Dec 23.

Blocked randomization with randomly selected block sizes.

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Center for Health Disparities Research and the Department of Public Health, Brody School of Medicine, East Carolina University, Physicians Quadrangle, Greenville, NC 27858, USA.


When planning a randomized clinical trial, careful consideration must be given to how participants are selected for various arms of a study. Selection and accidental bias may occur when participants are not assigned to study groups with equal probability. A simple random allocation scheme is a process by which each participant has equal likelihood of being assigned to treatment versus referent groups. However, by chance an unequal number of individuals may be assigned to each arm of the study and thus decrease the power to detect statistically significant differences between groups. Block randomization is a commonly used technique in clinical trial design to reduce bias and achieve balance in the allocation of participants to treatment arms, especially when the sample size is small. This method increases the probability that each arm will contain an equal number of individuals by sequencing participant assignments by block. Yet still, the allocation process may be predictable, for example, when the investigator is not blind and the block size is fixed. This paper provides an overview of blocked randomization and illustrates how to avoid selection bias by using random block sizes.


blocked randomization; random block sizes; randomized clinical trial

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