1. Department of Epidemiology and Biostatistics; School of Rural Public Health,Texas A&M Health Science Center; College Station TX 77843 U.S.A.
2. Department of Statistics; Texas A&M University; College Station TX 77843 U.S.A.
3. Division of Biostatistics and Epidemiology, Department of Public Health; Weill Medical College of Cornell University; New York NY 10065 U.S.A.