[1] Univ N Carolina, Sch Publ Hlth, Dept Epidemiol, Chapel Hill, NC USA
[2] Univ British Columbia, Fac Med, Dept Hlth Care Epidemiol, Vancouver, BC V5Z 1M9, Canada
[3] Univ N Carolina, Sch Med, Div Pediat Hematol Oncol, Chapel Hill, NC USA
[4] Univ Texas, MD Anderson Canc Ctr, Dept Epidemiol, Houston, TX 77030 USA
[5] Univ Florida, Sch Med, Dept Hlth Policy & Epidemiol, Gainesville, FL USA