Hospital versus population referents in two case-referent studies.

In case-referent studies, the choice between hospital and population referents involves the consideration of nonresponse, information bias, and selection bias. A case-referent study of astrocytoma and one of pancreatic cancer used both hospital and population referents. In the present report the two series of referents were compared with regard to potential sources of systematic errors. There were differences in reported exposure between the hospital and population referents. These differences seemed to be due to differences in exposure experience ("selection bias") rather than to differences in the quality of reported information ("information bias"). This conclusion was based on two findings. First, in the comparison between the hospital and population referents, similar differences were found in the reported exposure for "hard" questionnaire items as for "soft" questionnaire items. Second, in a comparison between reported radiographs and radiographs according to medical records, the same differences were found for the two series of referents, although there was considerable underreporting of such exposure in both series. Nonresponse rates were somewhat higher among the population referents than among the hospital referents.