Xiao Liu และ Peraphon Sophatsathit. (2013) EMPIRICAL STUDY OF SOURCE LEVEL DIFFICULTY. Chulalongkorn University:ม.ป.ท.Xiao Liu และ Peraphon Sophatsathit. 2013. EMPIRICAL STUDY OF SOURCE LEVEL DIFFICULTY. ม.ป.ท.:Chulalongkorn University;Xiao Liu และ Peraphon Sophatsathit. EMPIRICAL STUDY OF SOURCE LEVEL DIFFICULTY. ม.ป.ท.:Chulalongkorn University, 2013. Print.