1. Department of Mathematical Statistics; University of the Free State; 339, Bloemfontein 9300 South Africa
2. ITMO University; St. Petersburg Russia
3. Max Planck Institute for Demographic Research; Konrad-Zuse-Strasse 1,18057 Rostock Germany
4. Max-Planck Odense Center on the Biodemography of Aging; Odense Denmark