[1] Childrens Hosp New Orleans, Res Inst Children, New Orleans, LA 70118 USA
[2] Louisiana State Univ, Hlth Sci Ctr, Dept Pediat, New Orleans, LA USA
[3] George Washington Univ, Sch Med, Dept Epidemiol & Biostat, Washington, DC USA
[4] George Washington Univ, Sch Med, Dept Pediat, Washington, DC USA
[5] Childrens Natl Med Ctr, Childrens Res Inst, Biostat & Informat Unit, Washington, DC 20010 USA