Hai Linh, V. T. “Application of Importance - Performance Analysis Model in Evaluation Service Quality of Diving Tour Operator in Nha Trang (Viet Nam)”. AU-GSB E-JOURNAL, vol. 8, no. 2, June 2016, p. 42, http://www.assumptionjournal.au.edu/index.php/AU-GSB/article/view/2223.