1. Biometrics and Statistics Unit; International Maize and Wheat Improvement Center (CIMMYT); Apdo. Postal 6-641 06600 México D.F. México
2. Dep. of Biostatistics; Univ. of Alabama at Birmingham; Ryals Public Health Bldg. 443 Birmingham AL 35294
3. Dep. of Dairy Science; Univ. of Wisconsin; Madison WI 53706