1. Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, Washington 98195;
2. Division of Epidemiology, Department of Health Research and Policy, Stanford University School of Medicine, Stanford, California 94305;
3. Department of Environmental Health, School of Public Health and Community Medicine, University of Washington, Seattle, Washington 98195;
4. Department of Neurology, University of Washington, Seattle, Washington 98195;