Robert Holzmann, et al. “On the Heterogeneity in Longevity Among Socioeconomic Groups: Scope, Trends, and Implications for Earnings-Related Pension Schemes”. Global Journal of Human-Social Science, vol. 17, no. E1, Jan. 2017, pp. 31-57, https://socialscienceresearch.org/index.php/GJHSS/article/view/1961.