1. Biostatistics Branch, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709;
2. Unit of Epidemiology for Cancer Prevention, International Agency for Research on Cancer, and Institut National de la Santé et la Recherche Médicale, Lyon Cédex 08, 69372 France;
3. Department of Biometry and Epidemiology, Hollings Cancer Center, Medical University of South Carolina, Charleston, South Carolina 29425;
4. World Resources Institute, Washington, DC 20006;