Smutkupt, Srobol. “Applying Lean Six Sigma to Improve Telephone Bill Payment: A Case Study of A Real Estate Developer”. NIDA Case Research Journal 14, no. 2 (January 31, 2023): 87–109. accessed September 28, 2026. https://so04.tci-thaijo.org/index.php/NCRJ/article/view/263605.