Yatjaroen, J., S. Lertputtarak, and D. Samokin. “Service Quality Improvement of Shuttle Bus: Case Study Sapboon Transport and Service Company Limited”. KKBS Journal of Business Administration and Accountancy, vol. 5, no. 1, May 2021, pp. 89-112, https://so04.tci-thaijo.org/index.php/kkbsjournal/article/view/243328.