1. Department of Occupational and Environmental Health, University of Iowa, Iowa City, Iowa, USA
2. Epidemiology Branch, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, USA
3. Department of Epidemiology, and
4. Department of Biostatistics, University of Iowa, Iowa City, Iowa, USA
5. National Cancer Institute, National Institutes of Health, Department of Health and Human Services, Rockville, Maryland, USA