A slight mystery surrounds the clinical trial that this special issue of the BMJ commemorates.1 It is now widely recalled that pulmonary tuberculosis patients in the Medical Research Council (MRC) trial were allocated to a streptomycin treatment group and a control group by a process using random sampling numbers and sealed envelopes (box). The editorial that introduced the MRC's report to BMJ readers on 30 October 1948 called attention to this new scheme, distinguishing it from the older practice of taking alternate cases in order of admission to hospital as the method of creating a control group.2 The statistician involved, Professor (later Sir) Austin Bradford Hill, had been promoting the use of random allotment since before the second world war.3 Remarkably, however, the word “random” appeared nowhere in the MRC's files on streptomycin for 1946. During that year, the now famous scheme was explicitly mentioned in a single letter. So why did the MRC use randomisation in this clinical trial?

Any answer is tentative as contemporary evidence about the reasoning of the members of the MRC's committee is thin. We can make some inferences …