1. World Health Organization. Global health and aging [Internet]. Geneva, Switzerland. World Health Organization. 2011. cited at 2015 Oct 1. Available from: www.who.int/ageing/publications/global_health/en/
2. National Council on Aging. Health aging facts [Internet]. Arlington (VA). National Council on Aging. c2015. cited at 2015 Oct 1. Available from: https://www.ncoa.org/news/resources-for-reporters/get-the-facts/healthy-aging-facts/
3. Statistica. Statistics and facts about U.S. physicians [Internet]. New York (NY). Statistica. c2015. cited at 2015 Oct 1. Available from: http://www.statista.com/topics/1244/physicians/
4. World Health Organization. Country statistics: United States of America [Internet]. Geneva, Switzerland. World Health Organization. c2015. cited at 2015 Oct 1. Available from: http://www.who.int/countries/usa/en/