1. The authors are at the Center on Aging and Health and the Division of Geriatric Medicine and Gerontology at the Johns Hopkins University School of Medicine, Baltimore, MD 21224, USA (L.P.F. and J.D.W.), the National Institute on Aging, Bethesda, MD 20892, USA (E.C.H.), the Division of Geriatric Medicine at the University of Pittsburgh, Pittsburgh, PA 15213, USA (A.N. and S.S.), the Laboratory of Epidemiology, Demography, and Biometry at the National Institute on Aging, Bethesda, MD 20814, USA (J.M.G...