[1] Johns Hopkins Univ, Sch Med, Dept Neurol, Baltimore, MD 21205 USA
[2] Univ N Carolina, Res Triangle Inst Int, Chapel Hill, NC USA
[3] Univ N Carolina, Dept Biostat, Chapel Hill, NC USA
[4] Univ Texas Hlth Sci Ctr Houston, Houston, TX USA
[5] Univ Minnesota, Sch Publ Hlth, Div Epidemiol & Community Hlth, Minneapolis, MN USA
[6] Johns Hopkins Bloomberg, Dept Epidemiol, Sch Publ, Baltimore, MD USA