1. Morrow-Howell N. Volunteering in later life: research frontiers. J Gerontol B Psychol Sci Soc Sci. 2010;65(4):461–9.
2. Census and Statistics Department. Hong Kong population projections; 2017. Available at:
https://www.statistics.gov.hk/pub/B1120015072017XXXXB0100.pdf
. Accessed 19 Jan 2019.
3. Agency for Volunteer Service. Survey on volunteering in Hong Kong 2009; 2010. Available at:
http://www.volunteerlink.net/datafiles/A034.pdf
. Accessed 19 Jan 2019.
4. Bureau of Labor Statistics U.S. Department of Labor. Volunteering in the United States; 2015. Available at:
https://www.bls.gov/news.release/pdf/volun.pdf
. Accessed 19 Jan 2019.
5. Hoffmann E, Romeu Gordo L, Nowossadeck S, Simonson J, Tesch-Römer C. Living situations of older people in Germany (3rd edition, DZA-fact sheet) Berlin: Deutsches Zentrum für Altersfragen [German Centre of Geronotology]; 2017. Available at:
http://nbn-resolving.de/urn:nbn:de:0168-ssoar-52367-9
. Accessed 19 Jan 2019.