The Book of Why: The New Science of Cause and Effect (Penguin Science)

by Judea Pearl

Doll and Hill realized that if there were hidden biases in the case-control studies, mere replication would not overcome them. Thus, in 1951 they began a prospective study, for which they sent out questionnaires to 60,000 British physicians about their smoking habits and followed them forward in time. (The American Cancer Society launched a similar and larger study around the same time.) Even in just five years, some dramatic differences emerged. Heavy smokers had a death rate from lung cancer twenty-four times that of nonsmokers. In the American Cancer Society study, the results were even grimmer: smokers died from lung cancer twenty-nine times more often than nonsmokers, and heavy smokers died ninety times more often. On the other hand, people who had smoked and then stopped reduced their risk by a factor of two. The consistency of all these results—more smoking leads to a higher risk of cancer, stopping leads to a lower risk—was another strong piece of evidence for causality. Doctors call it a “dose-response effect”: if substance A causes a biological effect B, then usually (though not always) a larger dose of A causes a stronger response B.