1. Division of Epidemiology, Statistics and Prevention Research; Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH; Rockville MD U.S.A.
2. Department of Statistics; University of Connecticut; Storrs CT 06269 U.S.A.
3. Department of Biostatistics; University of North Carolina at Chapel Hill; Chapel Hill NC 27599 U.S.A.
4. Clinical Biostatistics; Merck Research Laboratories; Rahway NJ U.S.A.