[1] North West Univ, Populat Studies & Demog, ZA-2790 Mafikeng, South Africa
[2] Univ Ibadan, Coll Med, Dept Epidemiol & Med Stat, Ibadan 200212, Oyo, Nigeria
[3] Lilongwe Univ Agr & Nat Resources, Dept Agr & Appl Econ, POB 219, Lilongwe 265, Malawi
[4] Northwestern Univ, Feinberg Sch Med, Chicago, IL 60611 USA