1. Angola Oil and Gas Directory 2006. NDR Service, Paris. (http://www.omnimap.com/catalog/int/angola-gas.htm).
2. Banco Nacional de Angola Statistics (available http://www.bna.ao/default.aspx?c=300).
3. Robust convex optimisation;Ben-Tal;Mathematics of Operation Research,1998
4. An Introduction to Efficiency and Productivity Analysis;Coelli,1998
5. Bootstrap methods: another look at the jacknife;Efron;Annals of Statistics,1979