1. Center for Health Policy Research and Education, Box 90253, Duke University, Durham, North Carolina 27708, USA
2. Center for Health Policy Research and Education, Box 90253, Duke University, Durham, North Carolina 27708, USA, Department of Economics, Duke University, Durham, North Carolina 27708, USA, Fuqua School of Business, Duke University, Durham, North Carolina 27708, USA
3. Center for Health Policy Research and Education, Box 90253, Duke University, Durham, North Carolina 27708, USA, Department of Medicine (Neurology), Duke University, Durham, North Carolina 27708, USA, Division of Neurology, Durham Department of Veteran's Affairs Medical Center, Durham, North Carolina, 27710, USA
4. Center for Health Policy Research and Education, Box 90253, Duke University, Durham, North Carolina 27708, USA, Department of Economics, Duke University, Durham, North Carolina 27708, USA