1. About us: Mission;Duolingo,2020
2. From game design elements to gamefulness: Defining gamification;Deterding,2011
3. Strengthening gamification studies: Current trends and future opportunities of gamification research;Rapp;Int. J. Hum. Comput. Stud.,2019
4. Gamification market by deployment (on premises, cloud based, and hybrid), enterprise size (small and medium enterprises, and large enterprises), end user, and geography (North America, Europe, APAC, and row)-global forecast up to 2025;Research,2019
5. L. Piras, et al., Gamification Solutions for Software Acceptance: A Comparative Study of Requirements Engineering and Organizational Behavior Techniques, in: 11th International Conference on Research Challenges in Information Science, RCIS, 2017.