Differences in risk estimations from a hospital and a population-based case-control study.

A hospital-based and population-based case-control study of cervical cancer (in situ and invasive) was conducted in urban Utah to determine if methods of respondent selection affect estimates of risk for variables thought to be associated with the disease. Population cases (N = 409) and cases from two large hospitals (N = 124) were identified through the Utah Cancer Registry. Population-based controls (N = 379) were identified through random-digit dialing; hospital-based controls (N = 150) with gynaecological disorders other than cancers and elective abortions were chosen from the same hospitals as the cases for the hospital study. Both control groups were frequency matched to cases by age. Approximately 79% of the identified cases and 85% of the selected controls completed interviews conducted in their homes. Most risk estimates were lower in the hospital-based study because of the more case-like attributes of this group. Stratified analysis for social class led to adjusted risk estimates which were lower than the unadjusted risk estimates for the population-based study, but not for the hospital-based study. The close social class matching in the hospital-based study seems to have led to concurrent overmatching on other risk factors since many of these are closely related to social class. Findings are discussed in terms of implications for case-control study design.