Gilvan Guedes Demography Department - Universidade Federal de Minas Gerais   Av. Antônio Carlos, 6627 Pampulha Belo Horizonte, Minas Gerais 31270-901 Brazil   gilvan@gilvanguedes.com   www.gilvanguedes.com           JEL Areas: C1 : Econometric and Statistical Methods: General C4 : Econometric and Statistical Methods: Special Topics