[1] Univ Pittsburgh, Dept Epidemiol, Pittsburgh, PA 15261 USA
[2] Univ Pittsburgh, Dept Neurol Surg, Pittsburgh, PA 15260 USA
[3] Univ Pittsburgh, Dept Psychiat, Pittsburgh, PA USA
[4] Univ Pittsburgh, Dept Bioengn, Pittsburgh, PA USA
[5] NIA, Lab Epidemiol Demog & Biometry, Bethesda, MD 20892 USA