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Shortly after the end of the Second World War, Professor A. Bradford Hill and Dr (now Sir) Richard Doll published the first of a series of papers leading to the in- escapable conclusion that cigarette smoking was the major factor in the rising incidence of lung cancer. They began with a retrospective study; that is, they investigated a large number of patients with cancer of the lung and compared them with a carefully matched control group who did not have this form of cancer. After comparing a number of factors that might have a bearing on the cause of this disease, the only great difference to emerge was that the smoking habits of the two groups varied.

Only one in 200 male lung cancer patients were non-smokers, indicating smoking as the cause. The same sort of statistics appeared for women. Furthermore, there appeared to be a relationship between the risk of getting lung cancer and the number of cigarettes smoked. The problem with this kind of retrospective investigation is that a person’s memory, especially if he is ill, is inclined to be faulty and he might give the answer the questioner seems to want rather than the correct one. Bradford Hill and Doll therefore set up an investigation that would study the prospective health of smokers. They had 25.000 British doctors give details of their smoking habits as well as a variety of other relevant information. All the doctors were apparently well and had no reason to lie, and gave details of present smoking habits rather than the sometimes faulty recollections of past smoking.