1. Institute for Population and Development Studies, School of Public Policy and Administration, Xi’an Jiaotong University, Xi’an, Shaanxi Province, P. R. China;
2. Department of Gerontology, Simon Fraser University, Vancouver, BC, Canada;
3. Statistics Canada, Ottawa, ON, Canada;
4. Institute for Population and Development Studies, School of Public Policy and Administration, Xi'an Jiaotong University, Xi'an, Shaanxi Province, P. R. China