[1] George Washington Univ, Ctr Biostat, Rockville, MD USA
[2] Univ Michigan, Med Ctr, Ann Arbor, MI USA
[3] Medstar Hlth Res Inst, Washington, DC USA
[4] Univ Texas Hlth Sci Ctr San Antonio, San Antonio, TX 78229 USA
[5] Indiana Univ Sch Med, Indianapolis, IN USA
[6] Univ Pittsburgh, Pittsburgh, PA USA
[7] Ctr Dis Control, Atlanta, GA 30333 USA
[8] Johns Hopkins Univ, Baltimore, MD USA