Retrospective Study

Case–Control Study

As with other types of clinical investigation, the case–control study emerged from practices that originally belonged to the field of patient care. This form of disease investigation can be viewed as a combination of medical concepts (case definition, etiology, and a focus on the individual) and medical procedures (medical history taking, case series, and comparisons between the diseased and the healthy). The analytic form of the case–control study can be found in medical literature of the 19th century, but it did not appear to be viewed as a special or distinct methodology until the 20th century. The most fully developed investigation of [Page 1283]this type was Janet Lane-Claypon's 1926 study of breast cancer. This special field was crystallized in the years following World War II, when four case – control studies of smoking and lung cancer were published, and since then case–control study has been accepted as an approach to assessing disease etiology.

Retrospective Cohort Study

Retrospective cohort studies have almost as long a history as the prospective studies. The first study was described by Wade Hampton Frost in 1933, based on assessment of tuberculosis risk in the black population in Tennessee. Interviews identified 556 persons from 132 families, which created 10,000 person-years of observation. In the presence of family contact, the attack rate of tuberculosis, standardized for age, was found to be about double that in the absence of such a contact (12.9 per 1,000 vs. 6.8 per 1,000). Although unable, because of the small number of people under study, to provide evidence of the importance of family contacts in the spread of tuberculosis, this study revealed the way records of past events could be used for the study of public health and is notable in particular for its clear description of the way person-years at risk can be calculated and its success in gaining the cooperation of almost an entire population.

Earlier studies of significance include the 1920s study by Bradford Hill of company records of nickel refinery workers and pensioners and the risk of cancers in the lung and nose, and the success in reducing this risk through changes in the refinery process, the study by R. E. W. Fisher on coal-gas workers’ risk for lung cancer in 1920s, and many others.

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