During the twentieth century, Austin Bradford Hill researched diseases and their causes in England and developed the Bradford Hill criteria, which comprise the minimal requirements that must be met for a causal relationship to be established between a factor and a disease. Hill also suggested that researchers should randomize clinical trials to evaluate the effects of a drug or treatment by monitoring large groups of people. In addition, Hill advocated for case-control studies, in which researchers compare a group of people with a medical condition to a group without that condition to investigate the condition's possible causes. Hill's own work with clinical trials and case-control studies helped him prove that smoking caused lung cancer. The Bradford Hill criteria have also been used to establish causal links between factors and cancer, including reproductive cancers such as human papillomavirus that causes cervical cancer.

In 1965, Austin Bradford Hill published the article “The Environment and Disease: Association or Causation?” in the Proceedings of the Royal Society of Medicine. In the article, Hill describes nine criteria to determine if an environmental factor, especially a condition or hazard in a work environment, causes an illness. The article arose from an inaugural presidential address Hill gave at the 1965 meeting of the Section of Occupational Medicine of the Royal Society of Medicine in London, England. The criteria he established in the article became known as the Bradford Hill criteria and the medical community refers to them when determining whether an environmental condition causes an illness. The criteria outlined in “The Environment and Disease: Association or Causation?” help identify the causes of many diseases, including cancers of the reproductive system.

Richard Doll was an epidemiologist and public figure in the twentieth and early twenty-first centuries. Working primarily at the University of Oxford, in Oxford, England, Doll established a definitive correlation between cigarette smoking and lung cancer. Furthermore, Doll’s work helped legitimize epidemiology as a scientific discipline. Doll’s research also helped establish modern guidelines for oncological studies, as well as for contemporary and future research on the effect of smoking on pregnancy and fetal development. In addition to studying the health effects of smoking, Doll also studied cervical cancer and contraceptives.

From 1951 to 2001, researchers at the University of Oxford in Oxford, England, conducted the British Doctors’ Study, a study that examined the smoking habits, disease rates, and mortality rates of physicians in Britain. Two epidemiologists, scientists who study occurrence and distribution of disease, Richard Doll and Austin Bradford Hill, initiated the study, and statistician Richard Peto joined the team in 1971. The objective of the study was to assess the risks associated with tobacco use, and its relationship to lung cancer. The researchers tracked 34,439 male doctors practicing in Britain, and recorded smoking habits, development of diseases including lung cancer, other cancers, respiratory diseases, cardiovascular diseases, and mortality rates. The researchers published updated results every ten years. In 1980, Doll and other researchers concluded a related study on the smoking habits and disease rates of female doctors practicing in Britain. The results of both studies provided evidence that individuals who smoked tobacco were more likely to develop chronic respiratory illnesses, including lung cancer, and cardiovascular diseases.

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