Use and reporting of restricted randomization: a review

Abstract

Rationale, aims and objectives: Restricted randomization, such as blocking or minimization, allows for the creation of balanced groups and even distribution of covariates, but it increases the risk of selection bias and technical error. Various methods are available to reduce these risks but there is limited evidence about their current usage, and there are also indications that reporting of these methods may not be adequate. This review aims to identify how frequently different methods of restriction are being used and to assess the reporting of these methods against established reporting standards.

Methods: 82 reports of randomized controlled trial were reviewed. For each trial, the reported method of randomization was recorded and the reporting of randomization was assessed. Where the method of randomization was not clear from the main paper, protocols and other published materials were also reviewed, and authors were contacted for further information.

Results: For 11% of trials the method of randomization was not reported in either the paper or a published protocol, and in a further 39% of cases the report omitted key details so that the predictability of the method could not be evaluated. In total, 88% of trials appear to have used some form of restricted randomization, and all of those that report the exact methods used either blocking or minimization. 15% of trials reported using blocks of six or less and 4% used minimization with no random element reported, both of which are highly predictable.

Conclusion: Our results indicate that the majority of trials use some form of restriction, with many using relatively predictable methods that put them at greater risk of selection bias and technical error. Reporting of randomization methods often falls short of the minimum requirements set out by the CONSORT statement, leaving the reader unable to make an informed judgement about the risk of bias.

Keywords: randomization; randomized controlled trials; reporting; research methods; restricted randomization; stratification.