[1] Univ North Carolina Chapel Hill, Dept Biostat, 3104-E McGavran Greenberg Hall, Chapel Hill, NC 27515 USA
[2] Univ North Carolina Chapel Hill, Dept Nursing, 2005 Carrington Hall, Chapel Hill, NC 27515 USA
[3] Georgetown Univ, Dept Math & Stat, Washington, DC USA
[4] Harvard Sch Publ Hlth, Dept Biostat, Boston, MA USA
[5] Univ Massachusetts, Amherst, MA 01003 USA
[6] Massachusetts Gen Hosp, Dept Radiol, Boston, MA USA
[7] Massachusetts Gen Hosp, Dept Neurol, Boston, MA 02114 USA
[8] Harvard Univ, Cambridge, MA 02138 USA