1. Universidad de Quintana Roo, Chetumal, Quintana Roo, México
2. Biometrics and Statistics Unit, International Maize and Wheat Improvement Center (CIMMYT), 06600 México D. F., México
3. Facultad de Telemática, Universidad de Colima, C. P. 28040, Edo. de Colima, México
4. Colegio de Postgraduados, C. P. 56230 Montecillos, Edo. de México, México
5. Department of Epidemiology and Biostatistics, Michigan State University, East Lansing, Michigan 48824