Of course Hill knew that an RCT was impossible in this case, but he had learned the advantages of comparing a treatment group to a control group. So he proposed to compare patients who had already been diagnosed with cancer to a control group of healthy volunteers. Each group’s members were interviewed on their past behaviors and medical histories. To avoid bias, the interviewers were not told who had cancer and who was a control. The results of the study were shocking: out of 649 lung cancer patients interviewed, all but two had been smokers. This was a statistical improbability so extreme
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