COMPLETENESS AND ACCURACY OF INTERVIEW DATA FROM PROXY RESPONDENTS - DEMOGRAPHIC, MEDICAL, AND LIFE-STYLE FACTORS

To evaluate the quality of exposure data provided by proxy respondents , we used a dual interview protocol in a case-control study of subarac hnoid hemorrhage. All control subjects and their proxy respondents wer e interviewed (N = 283 control-proxy pairs), as were the cases who wer e able to provide their own information and their proxy respondents (N = 68 case-proxy pairs). The reliability of proxy derived data was exc ellent for demographic and body habitus measures (kappa or intraclass correlation range = 0.86-0.99), and all aspects of cigarette smoking h istory (range = 0.79-0.93). Proxy reliability was somewhat lower for q uestions regarding medications and hormone preparations (range = 0.55- 0.88), alcohol consumption (range = 0.52-0.82), and recreational physi cal activity (range = 0.55-0.67). Proxy reliability varied according t o the relationship of the proxy to the index subject. Relative to the index subjects, proxy respondents tended to underreport the presence o r level of exposure. For most exposures, adds ratios computed with pro xy derived data were similar in magnitude to odds ratios obtained with index subject data; important bias due to differential nonre sponse o r differential misclassification was suggested only for questions rega rding hormone replacement therapy. Epidemiologic studies that rely on proxy respondents may require more subjects to offset the effect of no ndifferential nonresponse and misclassification on the precision of ef fect estimates.