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Allocation concealment in randomised controlled trials: are we getting better?

BMJ 2016; 355 doi: (Published 17 November 2016) Cite this as: BMJ 2016;355:i5663
  1. Laura Clark, research fellow,
  2. Caroline Fairhurst, research fellow,
  3. David J Torgerson, director
  1. York Trials Unit, University of York, York YO10 5DD, UK
  1. Correspondence to: laura.clark{at}
  2. Accepted 18 October 2016

Laura Clark and colleagues assess the allocation concealment methods in a sample of randomised controlled trial publications

A robust randomised controlled trial (RCT) must use allocation concealment—that is, separate the act of randomisation from the person recruiting participants. Poor randomisation methods cause exaggerated treatment effects, are open to subversion by researchers or clinicians, and have a knock-on effect on systematic reviews.1 2 3

The CONSORT statement, which leading medical journals endorse, states that the method of allocation (comprising sequence generation, allocation concealment mechanism, and implementation) should be clearly described.4 Allocation concealment is dependent on the method of sequence generation as well as the concealment mechanism.

Almost a fifth of trials published in major medical journals in 2002 used inadequate concealment, and a quarter failed to describe how the allocation was concealed.2 Here we examine a sample of RCTs published in 2015 to see whether the situation has improved.

Defining inadequate allocation concealment

We searched four high impact medical journals (The BMJ, Journal of the American Medical Association (JAMA); the Lancet, and the New England Journal of Medicine (NEJM)) and found 79 RCTs published between June and August 2015. We extracted and judged their mechanism for allocation concealment, taking into consideration the study design, sequence generation method, and allocation concealment mechanism. We defined an inadequate process as one that used envelopes as the method of allocation concealment (box 1) or used stratified block randomisation by site with small block sizes as the sequence generation method (box 2), except in double blind trials. If insufficient detail was provided in the paper, we checked the protocol or emailed the authors.

Box 1: Sequentially numbered opaque sealed envelopes (SNOSE)

Envelopes containing the treatment allocation are opened by the recruiting clinician on participant enrolment. To be robust, the envelopes should be truly opaque, sequentially numbered, and opened in the correct order. The clinician …

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