1. Official Medicare Website. May 13, 2011. Available at: http://www.medicare.gov. Accessed June 15, 2011.
2. Medicare and You handbook 2012. Centers for Medicare & Medicaid Services. Available at: http://www.medicare.gov/Publications/Pubs/pdf/10050.pdf. Accessed September 7, 2011.
3. AARP Medicare Website. May 13, 2011. Available at: http://aarp.org/health/medicare-insurance/. Accessed June 15, 2011.
4. Preferred provider organizations: performance, problems, and promise;Gabel;Health Aff (Millwood),1985
5. The puzzling popularity of the PPO;Hurley;Health Aff (Millwood),2004