Song, Malin, and Shuhong Wang. “Environmental Efficiency Evaluation of China Based on a Kind of Congestion and Undesirable Output Coefficient”. Panoeconomicus 62, no. 4 (October 10, 2015): 453–468. Accessed May 2, 2024. http://www.panoeconomicus.org/index.php/jorunal/article/view/57.