Department of Statistics and Biostatistics Center, The George Washington University, 2140 Pennsylvania Avenue, N.W. Washington, D.C. 20052, USA. ylai@gwu.edu|Department of Statistics and Biostatistics Center, The George Washington University, 2140 Pennsylvania Avenue, N.W. Washington, D.C. 20052, USA. ylai@gwu.edu|Department of Statistics and Biostatistics Center, The George Washington University, 2140 Pennsylvania Avenue, N.W. Washington, D.C. 20052, USA. ylai@gwu.edu