[1] Univ N Carolina, Gillings Sch Global Publ Hlth, Dept Epidemiol, Chapel Hill, NC 27515 USA
[2] Johns Hopkins Univ, Sch Med, Dept Neurol, Cerebrovasc Div, Baltimore, MD 21205 USA
[3] Univ N Carolina, Gillings Sch Global Publ Hlth, Dept Biostat, Chapel Hill, NC USA
[4] Univ Arizona, Mel & Enid Zuckerman Coll Publ Hlth, Epidemiol & Biostat Div, Tucson, AZ USA