• Julie Pildal, An-Wen Chan, Asbjørn Hróbjartsson, Elisabeth Forfang, Douglas G Altman, and Peter C Gøtzsche.
• Nordic Cochrane Centre, H:S Rigshospitalet, DK-2100 Copenhagen, Denmark. jpl@dadlnet.dk
• BMJ. 2005 May 7; 330 (7499): 1049.

ObjectivesTo 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.DesignCohort study of 102 sets of trial protocols and corresponding publications.SettingProtocols of randomised trials approved by the scientific and ethical committees for Copenhagen and Frederiksberg, 1994 and 1995.Main Outcome MeasuresFrequency 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.Results96 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.ConclusionsMost 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.

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