1. Biostatistics and Epidemiology Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, Morgantown, WV 26505, USA
2. Department of Community Medicine, School of Medicine, West Virginia University, Morgantown, WV 26506, USA
3. College of Nursing, Washington State University, Spokane, WA 99210, USA
4. Department of Social and Preventive Medicine, School of Public Health and Health Professions, State University of New York at Buffalo, Buffalo, NY 14214, USA