1. Department of Epidemiology, University of Nebraska Medical Center, College of Public Health, Omaha, Nebraska.
2. Department of Health Services Research & Administration, University of Nebraska Medical Center, College of Public Health, Omaha, Nebraska.
3. University of Minnesota, Division of Epidemiology & Community Health, Minneapolis, Minnesota.
4. Department of Biostatistics, University of Nebraska Medical Center, College of Public Health, Omaha, Nebraska.