[1] Univ Washington, Inst Child Hlth, Dept Pediat, Seattle, WA 98115 USA
[2] Childrens Hosp & Reg Med Ctr, Seattle, WA USA
[3] RAND Corp, Santa Monica, CA USA
[4] Univ Calif Los Angeles, Dept Pediat, Los Angeles, CA 90024 USA
[5] Univ Calif Los Angeles, Dept Hlth Serv, Los Angeles, CA 90024 USA