[1] Univ Penn, Sch Med, Dept Emergency Med, Philadelphia, PA 19104 USA
[2] Univ Penn, Sch Med, Ctr Clin Epidemiol & Biostat, Philadelphia, PA 19104 USA
[3] Univ Penn, Leonard Davis Inst Hlth Econ, Philadelphia, PA 19104 USA
[4] Univ Pittsburgh, Sch Med, Dept Emergency Med, Pittsburgh, PA USA