Comparison of a Group with a Disease and a Control Group from the Same Families, in the Search for Possible Etiologic Factors

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Abstract
A method is presented for estimating the incidence rate of a disease in individuals with an antecedent factor, relative to the rate in their family relatives without the factor. The method utilizes cases of the disease, and their family relatives as controls. The estimate of the desired relative incidence rate equals the ratio: Pairs in which the case has the factor and the control does not Pairs in which the case does not have the factor but the control does. The method was applied to data from the Donora health survey, which observed an association between acute illness during the 1948 smog and subsequently reported heart disease. The rate of subsequent heart disease among individuals experiencing smog illness relative to the rate in their spouses without smog illness was estimated to be at least as great as the association in the total Donora population. The smog was interpreted as having acted as a screening test for subsequent heart disease.