1. Department of Social Medicine, University of North Carolina School of Medicine, 347A MacNider, 333 South Columbia Street, Chapel Hill, NC 27599-7240, USA
2. Departments of Population Health Sciences and Medicine, School of Medicine, Duke Clinical Research Institute, Duke University, 215 Morris Street, Durham, NC 27701
3. RTI International, 3040 E Cornwallis Rd, Research Triangle Park, NC 27709