%0 Letter
%T Re: "alternative data sources and discrepant results in case-control studies of estrogens and endometrial cancer".
%A Thomas DC
%J Am J Epidemiol
%V 113
%N 4
%D Apr 1981
%M 7211831
%F 5.363
%R 10.1093/oxfordjournals.aje.a113118
%X This letter argues how a previously published case-control (Horwitz et al.) study of estrogens and endometrial cancer illustrates how bias can arise in case-control studies when the proportion of interviews completed is related both to exposure and to disease.  In the Horwitz study, 2 series were reported: 1 obtained from interview and 1 obtained from medical data.  The odds ratios obtained from each of these series were similar, but a combined odds ratio produced very different results.  Horwitz attributed this to a confounding factor not taken into account, but the letter writer argues that bias was introduced because of the incompleteness of the interview series where both exposure and disease were related.  For example, in the conventional series, the interview rate in cases was higher for the exposed (91%) than the unexposed (61%), whereas in controls the reverse was true; in the alternative series, effect of modification was also present but to a much smaller degree, so the difference in odds ratio was correspondingly smaller.  In sum, merely taking a subject as exposed if so reported in either data source should be avoided to prevent bias.