DOGAN, N. O., and C. T. TUGCU. “Energy Efficiency in Electricity Production: A Data Envelopment Analysis (DEA) Approach for the G-20 Countries”. International Journal of Energy Economics and Policy, vol. 5, no. 1, Dec. 2014, pp. 246-52, https://econjournals.com/index.php/ijeep/article/view/1043.