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Comparison of descriptions of allocation concealment in trial protocols and the published reports: cohort study

BMJ 2005; 330 doi: (Published 05 May 2005) Cite this as: BMJ 2005;330:1049
  1. Julie Pildal, research fellow (jpl{at},
  2. An-Wen Chan, resident physician2,
  3. Asbjørn Hróbjartsson, senior research fellow1,
  4. Elisabeth Forfang, physician1,
  5. Douglas G Altman, director3,
  6. Peter C Gøtzsche, director1
  1. 1 Nordic Cochrane Centre, H:S Rigshospitalet, DK-2100 Copenhagen, Denmark,
  2. 2 Department of Medicine, University of Toronto,
  3. 3 Cancer Research UK Medical Statistics Group, Centre for Statistics in Medicine, Oxford OX3 7LF
  1. Correspondence to: J Pildal
  • Accepted 2 March 2005


Objectives To compare how allocation concealment is described in publications of randomised clinical trials and corresponding protocols, and to estimate how often trial publications with unclear allocation concealment have adequate concealment according to the protocol.

Design Cohort study of 102 sets of trial protocols and corresponding publications.

Setting Protocols of randomised trials approved by the scientific and ethical committees for Copenhagen and Frederiksberg, 1994 and 1995.

Main outcome measures Frequency of adequate, unclear, and inadequate allocation concealment and sequence generation in trial publications compared with protocols, and the proportion of protocols where methods were reported to be adequate but descriptions were unclear in the trial publications.

Results 96 of the 102 trials had unclear allocation concealment according to the trial publication. According to the protocols, 15 of these 96 trials had adequate allocation concealment (16%, 95% confidence interval 9% to 24%), 80 had unclear concealment (83%, 74% to 90%), and one had inadequate concealment. When retrospectively defined loose criteria for concealment were applied, 83 of the 102 trial publications had unclear concealment. According to their protocol, 33 of these 83 trials had adequate allocation concealment (40%, 29% to 51%), 49 had unclear concealment (59%, 48% to 70%), and one had inadequate concealment.

Conclusions Most randomised clinical trials have unclear allocation concealment on the basis of the trial publication alone. Most of these trials also have unclear allocation concealment according to their protocol.


  • Contributors All authors conceived and designed the study, analysed and interpreted the data, critically revised the manuscript and approved the final version of the manuscript. JP, AH, EF, and PCG acquired the data. PGC provided administrative support for the study. JP drafted the manuscript. She is guarantor for the paper and accepts full responsibility for the conduct of the study, had access to the data, and controlled the decision to publish.

  • Funding JP, AH, EF, and PCG are funded by Copenhagen Hospital Corporation. AWC was supported by the Rhodes Trust. DGA is supported by Cancer Research UK. The funding organisations had no role in any aspect of the study, the manuscript, or the decision to publish.

  • Conflict of interests None declared

  • Ethical approval Not required.

  • Accepted 2 March 2005
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