1. Department of Epidemiology; University of North Carolina at Chapel Hill; Campus Box 7435 27599-7435 Chapel Hill NC USA
2. Washington State Department of Health; Office of Epidemiology; 101 Israel Road SE 98504-7814 Olympia WA USA
3. Department of Epidemiology, Biostatistics, and Occupational Health; McGill University; 1020 Pine Ave H3A 1A2 West Montreal QC Canada
4. School of Community Health Sciences; University of Nevada; Mailstop 274 89557-0036 Reno NV USA
5. Department of Sociology; Duke University; 347 Sociology-Psychology Building 27708-0088 Durham NC USA