Alternative data sources and discrepant results in case-control studies of estrogens and endometrial cancer

In most case-control studies, little attention has been given to two features that can affect the odds ratio: 1) self-selection of the people who are available for interviews, and 2) disagreements between the data obtained by medical records and by direct conversation. The authors investigated these problems during two separate case-control studies of estrogens and endometrial cancer. When interviews were attempted with all patients, fewer controls than cases were available. In samples of patients from a tumor registry, more controls than cases had died before the interview could be solicited, and fewer control patients could be located. In patients sampled from a diagnostic test registry, more controls than cases refused participation. The interview data about estrogen usage disagreed with previously recorded medical information in 16% (52/324) of patients). Among persons marked as "non-users" in medical record data, more cases than controls were available for interview and thus for redesignation as estrogen users. The bias produces a possible artificial increase in the odds ratio when interview and medical record data are combined. Since patients available for interview were systematically biased in favor of having both estrogen use and endometrial cancer, the odds ratio for this association should be calculated separately for record and for interview data.