Takele, A. D., Y. G. Selassie, and S. Tekset. “Farmers’ Willingness to Pay for 2-Wheel Tractor Hiring Services in Northwestern Ethiopia: A Contingent Valuation Study”. Asian Journal of Agriculture and Food Sciences, vol. 6, no. 6, Dec. 2018, doi:10.24203/ajafs.v6i6.5592.