1. China Health and Retirement Longitudinal Study (CHARLS) website. Available: http://charls.ccer.edu.cn/en. Accessed 2014 Jan 8.
2. English Longitudinal Study of Ageing (ELSA) website. Available: http://www.ifs.org.uk/ELSA. Accessed 2014 Jan 8.
3. Health and Retirement Study (HRS) website. Available: http://hrsonline.isr.umich.edu/. Accessed 2014 Jan 8.
4. Korean Longitudinal Study of Ageing (KLoSA) website. Available: http://www.kli.re.kr/klosa/en/about/introduce.jsp. Accessed 2014 Jan 8.
5. Study on global AGEing and adult health (SAGE) website. Available: http://www.who.int/healthinfo/sage/en/. Accessed 2014 Jan 8.