Wang, Chia Nan, and Xuan Tho Nguyen. “A Hybrid Approach for Measuring Productivity in the Global Fitness Industry by Using Grey and DEA Model”. American Scientific Research Journal for Engineering, Technology, and Sciences 38, no. 1 (November 15, 2017): 33–47. Accessed April 28, 2024. https://asrjetsjournal.org/index.php/American_Scientific_Journal/article/view/3591.