[1] Univ Navarra, Edificio Bibliotecas, E-31080 Pamplona, Spain
[2] Inst Cultura & Soc, Navarra Ctr Int Dev, Edificio Bibliotecas, Pamplona 31080, Spain
[3] UCL, Dept Econ, Gower St, London WC1E 6BT, England
[4] Inst Fiscal Studies, Ctr Microdata Methods & Practice, 7 Ridgmount St, London WC1E 7AE, England
[5] Bank Italy, Trento Branch, Piazza A Vittoria 6, I-38122 Trento, Italy
[6] Minist Hlth, Natl Malaria Control Program, POB 212, Asmera, Eritrea
[7] Tulane Univ, Sch Publ Hlth & Trop Med, Dept Trop Med, 1440 Canal St,Suite 2301, New Orleans, LA 70112 USA