[1] Univ Penn, Dept Biostat & Epidemiol, Philadelphia, PA 19104 USA
[2] Univ Penn, Sch Med, Ctr Clin Epidemiol & Biostat, Philadelphia, PA 19104 USA
[3] Univ Maryland, Div Gerontol, Baltimore, MD USA
[4] Univ Penn, Sch Med, Dept Dermatol, Philadelphia, PA 19104 USA