1. Division of Clinical and Translational Sciences, Department of Internal Medicine; University of Texas Health Science Center at Houston; Houston, Texas 77030 U.S.A.
2. Department of Biostatistics; Indiana University, Fairbanks School of Public Health and School of Medicine; Indianapolis, Indiana 46202 U.S.A.
3. School of Mathematical Sciences; Shanghai Jiao Tong University; Shanghai 200240 P. R. China
4. Department of Biostatistics; St. Jude Children's Research Hospital; Memphis, Tennessee 38105 U.S.A.
5. Department of Statistics; University of Missouri; Columbia, Missouri 65211 U.S.A.
6. School of Mathematics; Jilin University; Changchun 130012 P. R. China
7. Department of Epidemiology and Cancer Control; St. Jude Children's Research Hospital; Memphis, Tennessee 38105 U.S.A.