1. Department of Epidemiology and Biostatistics, University of Maryland, College Park, MD 20742, USA
2. Office of Research and Evaluation, Office of Planning, Analysis and Evaluation, Health Resources and Services Administration, U.S. Department of Health and Human Services, 5600 Fishers Lane, Rockville, MD 20857, USA
3. Office of Quality and Data, Bureau of Primary Health Care, Health Resources and Services Administration, U.S. Department of Health and Human Services, 5600 Fishers Lane, Rockville, MD 20857, USA