Hidayat, M., R. Darwin, and M. F. Hadi. “Inequality of Interregional Development in Riau Indonesia; Panel Data Regression Approach”. International Journal of Economics and Financial Issues, vol. 8, no. 5, Sept. 2018, pp. 184-9, https://www.econjournals.com/index.php/ijefi/article/view/6919.