1. Centers for Disease Control and Prevention, National Center for Health Statistics, National Hospital Ambulatory Medical Care Survey, 2002.
2. Center for Medicare and Medicaid Services (CMS), Washington, DC.
3. Current procedural terminology, (CPT),2004
4. PubMed, U.S. National Library of Medicine's digital archive of life sciences journal literature.
5. Google search engine, Google Inc, Mountain View, CA, 94043.