1. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, U.S.A.
2. Rho Federal Systems Division, Inc., Chapel Hill, North Carolina, U.S.A.
3. Division of Intramural Research, National Institute for Environmental Health Sciences, Research Triangle Park, North Carolina, U.S.A.
4. Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, Quebec, Canada