Littenberg-Tobias, Joshua, José A. Ruipérez-Valiente, and Justin Reich. “Studying Learner Behavior in Online Courses With Free-Certificate Coupons: Results From Two Case Studies”. The International Review of Research in Open and Distributed Learning 21, no. 1 (January 1, 2020): 1-22. Accessed June 12, 2021. http://www.irrodl.org/index.php/irrodl/article/view/4564.