Impact evaluation protocols for agricultural projects: the case of yam improvement for income and food security in West Africa - Mignouna, Djana B.; Akinola, Adebayo A.; Abdoulaye, Tahirou; Alene, A.; Maroya, N.; Aighewi, B.A.; Wobill, T.; Asiedu, Robert (International Institute of Tropical Agriculture, 2016)