Malawal Phattarapong, Wongchai Anupong, and Lin Yi-Chia. “Cost Efficiency of Rubber Farmers in the Upper Northern Provinces of Thailand”. International Journal of Advanced Science and Technology, Vol. 29, no. 9s, May 2020, pp. 4411 -8, http://sersc.org/journals/index.php/IJAST/article/view/16917.