1. V. Pittman, “Rivalry for respectability: Collegiate and proprietary correspondence programs”, Second American symposium on research in distance education. 1991, 183–184.
2. N. U. Yarychev and A. U. Mentsiev, J. Phys.: Conf. Ser. 1691 012128 (2020).
3. G. Shah, International Council for Distance Education Bulletin. 19 9–14 (1989).
4. Distance Education for Primary and Secondary Teacher Training in Turkey
5. C. N. Gunawardena and M. S. McIsaac, Handbook of research on educational communications and technology. 361–401 (2013).