Mebratu, A. A. ., F. . Leykun, and M. . Lakehal-Ayat. “Determinant of Tax Revenue Effort in Sub-Saharan African Countries: A Stochastic Frontier Analysis”. International Journal of Sustainable Development & World Policy, vol. 9, no. 1, May 2020, pp. 47-71, doi:10.18488/journal.26.2020.91.47.71.