Medicare.Gov The Official Site For Medicare. How Medicare works with other insurance. https://www.medicare.gov/supplement-other-insurance/how-medicare-works-with-other-insurance/how-medicare-works-with-other-insurance.html. Accessed May 2016.
US Centers for Medicare & Medicaid Services. Health insurance plan & network types: HMOs, PPOs, and more. HealthCare.gov. https://www.healthcare.gov/choose-a-plan/plan-types/. Accessed May 2016.