1. CCTS, University of Texas Health Science Center at Houston, Houston, TX, USA
2. School of Mathematical Sciences, Beijing Normal University, Beijing, People's Republic of China
3. Biostatistics, St. Jude Children's Research Hospital, Memphis, TN, USA
4. Biostatistics, University of Texas MD Anderson Cancer Center, Houston, TX, USA
5. Departments of Epidemiology and Cancer Control, St. Jude Children's Research Hospital, Memphis, TN, USA