Littenberg-Tobias J, Ruipérez-Valiente JA, Reich J. Studying Learner Behavior in Online Courses With Free-Certificate Coupons: Results From Two Case Studies. IRRODL [Internet]. 2020 Jan. 1 [cited 2024 Nov. 23];21(1):1-22. Available from: https://www.irrodl.org/index.php/irrodl/article/view/4564