1. Biometrics and Statistics UnitInternational Maize and Wheat Improvement Center (CIMMYT) Lisboa 27, Apdo. Postal 6‐641 06600 Mexico D.F. Mexico
2. Dep. of Agronomy and Dep. of StatisticsUniv. of Kentucky Lexington KY 40546‐0091
3. Dep. of Plant AgricultureUniv. of Guelph Guelph ON Canada N1G2W1