[1] UN, Dept Econ & Social Affairs, Populat Div, New York, NY 10017 USA
[2] Catholic Univ Louvain, Ctr Rech Demog & Soc, Louvain La Neuve, Belgium
[3] Columbia Univ, Mailman Sch Publ Hlth, Dept Populat & Family Hlth, New York, NY USA
[4] WHO, CH-1211 Geneva, Switzerland