Wongpichet, S., and C. Watyotha. “The Suitability of The Utilization of Small Second-hand Agricultural Tractor : Case Study in Khon Kaen Province”. Engineering and Applied Science Research, vol. 31, no. 2, Mar. 2013, pp. 123-32, https://ph01.tci-thaijo.org/index.php/easr/article/view/6707.