[1] UNIV PENN, SCH MED, DEPT PEDIAT, PHILADELPHIA, PA 19104 USA
[2] UNIV PENN, SCH MED, DEPT MED, GEN INTERNAL MED SECT, PHILADELPHIA, PA 19104 USA
[3] UNIV PENN, SCH MED, DEPT MED, CLIN EPIDEMIOL UNIT, PHILADELPHIA, PA 19104 USA
[4] UNIV PENN, WHARTON SCH, DEPT HLTH CARE SYST, PHILADELPHIA, PA 19104 USA
[5] UNIV PENN, LEONARD DAVIS INST HLTH CARE ECON, PHILADELPHIA, PA 19104 USA
[6] UNIV TORONTO, DEPT PEDIAT, DIV EMERGENCY MED, TORONTO M5S 1A1, ONTARIO, CANADA
[7] HARVARD UNIV, SCH MED, DEPT PEDIAT, DIV EMERGENCY MED, BOSTON, MA 02115 USA