Problems in the use of dead controls in case-control studies. II. Effect of excluding certain causes of death

A recent comparison of 493 dead and 697 living controls from a case-control study of cancer in the Minneapolis-St. Paul area showed that the dead controls of both sexes were reported to have been significantly heavier consumers of cigarettes, hard liquor, beer, and certain drugs, and to have had more adulthood diseases than living controls. The present analysis examines the effect of excluding causes of death associated with those exposures found in excess in the dead controls. Exclusion of individuals with smoking-related causes of death reduced but did not eliminate the excess of cigarette smokers among the dead controls. Deletion of individuals with alcohol-related causes of death only slightly reduced the excess among dead controls. Adjustment for cigarette smoking, however, nearly eliminated the association with alcohol consumption, particularly among males. For certain adulthood diseases and medications, the exclusion of individuals with exposure-associated causes of death also virtually eliminated the excesses found in the dead controls when compared with the living controls. Thus, it appears that even after extensive exclusion of smoking-related causes of death, the association between dead controls and cigarette smoking still remains, and the use of dead controls in case-control studies where cigarette smoking is the risk factor being evaluated may lead to a biased underestimated of risk. For the other exposures found in significant excess among the dead controls, the exclusion of exposure-related causes of death and proper adjustment for confounders may eliminate much or all of the excess.