About the authors REGINA C. ELANDT-JOHNSON has been Professor of Biostatistics at the University of North Carolina at Chapel Hill since 1964. She is the author of Probability Models and Statistical Methods in Genetics (Wiley, 1971). Dr. Elandt-Johnson received her Ph.D. in statistics from Poznan Agricultural University in 1957. NORMAN L. JOHNSON is Alumni Distinguished Professor at the University of North Carolina at Chapel Hill. Dr. Johnson served as Chairman of the Fisher Memorial Lecture Committee, American Statistical Association from 1976 until 1979. He is co-author of Distributions in Statistics (Wiley, 1969-1972); URN Models and their Applications (Wiley, 1977); and Statistics and Experimental Design in Engineering and Physical Sciences (Wiley, 1977). Dr. Johnson received his D.Sc. in statistics from University College, London in 1963.