1. Statistical Center for HIV/AIDS Research and Prevention, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA.
2. Center for Studies in Demography and Ecology, University of Washington, Seattle, WA98195, USA.
3. Department of Epidemiology, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
4. International Health Program, University of Washington, Seattle, WA 98195, USA.
5. Clinical Research Division, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA.