[1] Univ Utah, Dept Internal Med, Div Epidemiol, Williams Bldg,295 Chipeta Way, Salt Lake City, UT 84108 USA
[2] Univ Utah, Dept Pediat, Salt Lake City, UT 84108 USA
[3] Univ Calif Los Angeles, Dept Biostat, Los Angeles, CA 90095 USA
[4] Univ Utah, Dept Orthopaed, Salt Lake City, UT 84108 USA
[5] Univ Utah, Dept Populat Hlth Sci, Salt Lake City, UT 84108 USA