[1] Univ Maryland, Sch Med, Dept Epidemiol & Publ Hlth, Baltimore, MD 21201 USA
[2] Univ Iowa, Hosp & Clin, Iowa City, IA 52242 USA
[3] Univ Iowa, Coll Publ Hlth, Dept Epidemiol, Iowa City, IA 52242 USA
[4] Univ Iowa, Hosp & Clin, Dept Internal Med, Iowa City, IA 52242 USA
[5] Univ Iowa, Hosp & Clin, Dept Pediat, Iowa City, IA 52242 USA
[6] Univ Iowa, Hosp & Clin, Dept Orthoped, Iowa City, IA 52242 USA