Researchers often recruit proxy respondents, such as relatives or caregivers, for epidemiologic studies of older adults when study participants are unable to provide self-reports (eg, because of illness or cognitive impairment). In most studies involving proxy-reported outcomes, proxies are recruited only to report on behalf of participants who have missing self-reported outcomes; thus, either a proxy report or participant self-report, but not both, is available for each participant. When outcomes are binary and investigators conceptualize participant self-reports as gold standard measures, substituting proxy reports in place of missing participant self-reports in statistical analysis can introduce misclassification error and lead to biased parameter estimates. However, excluding observations from participants with missing self-reported outcomes may also lead to bias. We propose a pattern-mixture model that uses error-prone proxy reports to reduce selection bias from missing outcomes, and we describe a sensitivity analysis to address bias from differential outcome misclassification. We perform model estimation with high-dimensional (eg, continuous) covariates using propensity-score stratification and multiple imputation. We apply the methods to the Second Cohort of the Baltimore Hip Studies, a study of elderly hip fracture patients, to assess the relation between type of surgical treatment and perceived physical recovery. Simulation studies show that the proposed methods perform well. We provide SAS programs in the eAppendix (http://links.lww.com/EDE/A646) to enhance the methods’ accessibility.
From the aDepartment of Epidemiology and Public Health, University of Maryland School of Medicine; bNational Institute on Aging; cDepartment of Physical Therapy, University of Delaware; and dUniversity of Maryland School of Nursing, Baltimore, MD.
Submitted 24 April 2012; accepted 21 November 2012.
Supported by NIH grants K25 AG034216 (M.S.), R01 AG06322 (J.M.), and R01 AG09902 (J.M.).
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Correspondence: Michelle Shardell, University of Maryland School of Medicine, 660 West Redwood Street, Baltimore, MD 21201. E-mail: firstname.lastname@example.org.