1. He, W., Goodkind, D., Kowa, P.: An Aging World: 2015. International Population Reports. United States Census Bureau, Washington, DC (2015). https://www.census.gov/content/dam/Census/library/publications/2016/demo/p95-16-1.pdf
2. Statistics Canada: Population by Sex and Age Group. Statistics Canada, Ottawa (2015). http://www.statcan.gc.ca/tables-tableaux/sum-som/l01/cst01/demo10a-eng.htm
3. World Health Organization: World Health Statistics 2014. World Health Organization, Geneva, Switzerland (2014). http://apps.who.int/iris/bitstream/10665/112738/1/9789240692671_eng.pdf
4. Kaufman, D.: Aging well: can digital games help older adults? In: Bastiaens, T., Marks, G. (eds.) Proceedings of World Conference on E-Learning in Corporate, Government, Healthcare, and Higher Education 2013, pp. 1943–1949. AACE, Chesapeake (2013)
5. McPherson, M., Smith-Lovin, L., Brashears, M.E.: Social isolation in America: changes in core discussion networks over two decades. Am. Sociol. Rev. 71(3), 353–375 (2006)