1. Department of Medicine and Department of Epidemiology and Biostatistics, University of California, San Francisco, California 94143, USA
2. The UCSF Center for Vulnerable Populations, Zuckerberg San Francisco General Hospital, San Francisco, California 94110, USA;
3. Division of General Medicine and Clinical Epidemiology, School of Medicine, University of North Carolina, Chapel Hill, North Carolina 27599, USA
4. Cecil G. Sheps Center for Health Services Research, University of North Carolina, Chapel Hill, North Carolina 27599-7590, USA;