Alkaff, A., M. N. Qomarudin, S. D. Alkaff, and Y. Bilfaqih. “Modelling Online Course Services and Comparison of Its Major Providers”. International Journal of Emerging Technologies in Learning (iJET), vol. 13, no. 01, Jan. 2018, pp. pp. 65-81, doi:10.3991/ijet.v13i01.7528.