Intended for healthcare professionals

Analysis And Comment Research methods

Is restricted randomisation necessary?

BMJ 2006; 332 doi: (Published 22 June 2006) Cite this as: BMJ 2006;332:1506
  1. Catherine E Hewitt, PhD student1,
  2. David J Torgerson, director (
  1. 1York Trials Unit, Department of Health Sciences, University of York, York YO10 5DD
  1. Correspondence to: D Torgerson
  • Accepted 27 April 2006

Restrictions during randomisation make it easier for investigators to guess the next allocation. Statistical correction of any imbalance in confounders at the end of the study is equally accurate for most trials and would be safer

Randomised controlled trials commonly use some form of restriction when allocating participants—for example, blocking, stratification, or minimisation.1 2 The main reason is to achieve a better balance of known confounders. For an individual trial simple randomisation may lead to some chance imbalances on some variables. These imbalances are unimportant if the variables have a weak relation with the outcome. However, if by chance the groups differ at baseline on one or more confounding variables, the trial result may be misleading; this effect could be in either direction.

Stratification and minimisation are used to reduce these chance imbalances. Blocking needs to be used in combination with stratification to ensure that roughly equal numbers of participants are randomised to each of the treatments in the individual strata.1 Otherwise, it is statistically equivalent to using simple randomisation. However, using restricted randomisation can generate its own problems.

Problems with restricted randomisation

Introducing any form of restricted randomisation increases the risk of subversion (conscious or unconscious) and technical error. Most examples of known subversion relate to situations where allocation sequences are public knowledge or the concealment of the allocation is inadequate, such as using sealed envelopes that can be tampered with.3 4 However, this type of subversion is not specific to restricted randomisation.

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What will come next?


With restricted randomisation subversion remains possible even when adequate precautions have been taken to conceal the randomisation sequence.5 In an open trial, if we know the block size and we keep a record of previous allocations then we will always be certain about the last allocation in the block. Furthermore, …

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