1. Medicine and Social and Behavioral Sciences, Clinical Addiction Research and Education (CARE) Unit, Section of General Internal Medicine, Boston University Schools of Medicine and Public Health, Boston, Massachusetts 02118
2. Department of Mathematics, Smith College, Northampton, Massachusetts 01063
3. Medicine and Social and Behavioral Sciences, Clinical Addiction Research and Education (CARE) Unit, Section of General Internal Medicine, Boston University Schools of Medicine and Public Health, Boston, Massachusetts 02118
4. Division of General Medicine and Partners AIDS Research Center, Massachusetts General Hospital, Boston, Massachusetts 02114