1. Department of Family Medicine and Community Health, University of Massachusetts Medical School, Worcester, MA, USA
2. Department of Quantitative Methods, University of Massachusetts Medical School, Worcester, MA, USA
3. National Health Care for the Homeless Council, Nashville, TN, USA
4. Department of Family Medicine, David Geffen School of Medicine, and Department of Health Policy and Management, Fielding School of Public Health, University of California at Los Angeles, Los Angeles, CA, USA