1. Academy Information Disclosure Center (2015). The state of dropout students in cyber colleges in 2015. Retrieved August 16, 2016 from
http://www.academyinfo.go.kr/UIPISA/uipnh/unt/ipsrch/UntUntCprsSrchRdviewer.do
.
2. Ainoda, A., Onishi, H., & Yasuda, Y. (2005). Definitions and goals of self-directed in contemporary medical education literature. Annals of the Academy of Medicine, 34, 286–290.
3. Allen, E., & Seaman, J. (2007). Online nation: five years of growth in online learning. Needham, MA: The Sloan Consortium.
4. Allen, E., & Seaman, J. (2014). Grade change: Tracking online education in the United States. Retrieved January 18, 2017 from
http://www.onlinelearningsurvey.com/reports/gradechange.pdf
.
5. Bak, B., & Park, S. (2012). Development and validation of an academic stress scale. The Korean Journal of Educational Psychology, 26, 563–585.