1. Department of Epidemiology and Biostatistics, School of Public Health, Texas A&M University, College Station, Tx., U.S.A.
2. Department of Epidemiology and Biostatistics, College of Public Health, University of South Florida, Tampa, Fl., U.S.A.
3. Department of Biostatistics, Bioinformatics and Biomathematics, Georgetown University, Washington, D.C., U.S.A.