1. Department of Medicine, The Johns Hopkins Medical Institutions (A.R.C., G.S.W., L.P.F.), Baltimore, Maryland 21205
2. Department of Biostatistics, Johns Hopkins School of Hygiene and Public Health (K.B.-R.), Baltimore, Maryland 21205
3. Epidemiology, Demography, and Biometry Program, National Institute on Aging, National Institutes of Health (S.V.), Bethesda, Maryland 20892