1. Department of Biostatistics; The University of Texas M. D. Anderson Cancer Center; Houston; TX 77030; U.S.A.
2. Biostatistics Research Branch; National Institute of Allergy and Infectious Diseases; Bethesda; MD 20892; U.S.A.
3. Department of Mathematics and Statistics; McGill University; Montreal; QC; H3A 2K6; Canada