1. Department of Biostatistics; The University of Texas MD Anderson Cancer Center; Houston TX 77030 U.S.A.
2. School of Statistics; Xi'an University of Finance and Economics; Xi'an 710061 China
3. Department of Health Statistics; The Fourth Military Medical University; Xi'an 710032 China
4. Office of Biostatistics, Center for Drug Evaluation; China Food and Drug Administration (CFDA); Beijing 100038 China