Why transition from alternation to randomisation in clinical trials was made

doi:10.1136/bmj.319.7221.1372

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Iain Chalmers, director (ichalmers@cochrane.co.uk)

Cochrane Centre, Oxford OX2 7LG

EDITOR—It is time to put the record straight about ways of controlling selection bias when generating comparison groups in clinical trials. D'Arcy Hart has done history a great service by noting that Bradford Hill's motivation for replacing alternation with randomisation was “to better conceal the allocation schedule.”1 This is what Guy Scadding (who, with D'Arcy Hart, is the other surviving member of the team who designed the streptomycin trial) told Mike Clarke and me when we visited him on 10 June 1999, and what Bradford Hill told William Silverman and me when we visited him on 3 April 1982.

Bradford Hill's motivation for concentrating on the concealment of …