1. Epidemiology Branch and
2. Biostatistics Branch, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, USA
3. Social and Scientific Systems, Inc., Durham, North Carolina, USA
4. Westat, Durham, North Carolina, USA
5. Department of Environmental Health Sciences, School of Public Health, University of Michigan, Ann Arbor, Michigan, USA