1. Demography and Population Studies Programme, Schools of Public Health and Social Sciences, University of the Witwatersrand, Johannesburg 2000, South Africa
2. Centre for Social Development in Africa, University of Johannesburg, Johannesburg 2092, South Africa
3. Department of Demography and Social Statistics, Faculty of Social Sciences, Federal University Oye-Ekiti, Oye-Ekiti 371104, Nigeria