1. Center for Environmental and Resource Economic Policy, and Resources for the Future, Campus Box 8109, North Carolina State University, Raleigh, NC 27695.
2. Center for Health Policy, Law and Management, Terry Sanford Institute of Public Policy, Duke University, 122 Old Chemistry Building, Box 90253, Duke University, Durham, NC 27708.
3. Center for Health Policy, Law and Management and Department of Economics, Duke University, 125 Old Chemistry Building, Box 90253, Duke University, Durham, NC 27708.