Conrad, Dianne. “Cognitive, Instructional, and Social Presence As Factors in Learners’ Negotiation of Planned Absences from Online Study”. The International Review of Research in Open and Distributed Learning 10, no. 3 (June 26, 2009). Accessed October 21, 2019. http://www.irrodl.org/index.php/irrodl/article/view/630.