1. Biostatistics Unit, Group Health Research Institute, Seattle, WA 98101, USA.
2. Department of Biostatistics, University of Washington, Seattle, WA 98195, USA.
3. Group Health Research Institute, Seattle, WA 98101, USA.
4. Department of Medicine, University of Washington, Seattle, WA 98195, USA.
5. Department of Epidemiology, University of Washington, Seattle, WA 98195, USA.
6. Department of Health Services, University of Washington, Seattle, WA 98195, USA.