Use of twins as mutual proxy respondents in a case-control study of breastcancer: Effect of item nonresponse and misclassification

A case-control study of breast cancer in twins diagnosed before 1988 was us ed to characterize the effects on odds ratios when proxy responses from co- twins are used. North American disease-discordant pairs were ascertained th rough advertisements, and mailed questionnaires were returned from both mem bers of 671 pairs and from one member of 391 pairs. Biases from the proxy r esponse were attributed to nonresponse or misclassification. Nonresponse va ried according to type of exposure variable, depth of detail requested, joi nt exposure status of the pair, respondent's case-control status, zygosity, and social closeness of the pair. Misclassification was minimal, generally nondifferential, and a high degree of reliability between the proxy and se lf-report was indicated by the kappa statistic and the intraclass correlati on coefficient. By using double-respondent pairs, a method was developed to adjust proxy responses for both sources of bias. These adjustments resulte d in minor changes to the odds ratios for the variables studied (age at men arche, reproductive factors, and hormone use). A larger difference was obse rved between the odds ratios based on ail pairs and those based on double-r espondent pairs only. These findings demonstrate that, for these Variables in this population, twins are reliable proxies for each other and that resu lts from single-respondent pairs should be included.