1. Department of Statistics; Tianjin University of Finance and Economics; Tianjin 300222 China
2. Department of Biostatistics, Biomathematics and Bioinformatics; Georgetown University; DC 20057 Washington U.S.A.
3. Department of Statistics; The Pennsylvania State University, University Park; PA 16802 U.S.A.
4. LPMC and Institute of Statistics; Nankai University; Tianjin 300071 China