Baidoo, F., and J. B. Servo. “Lean and Six Sigma Improvement Concept: Automobile After-Sales Service Delays on Customer Satisfaction in Ghana”. AFRICAN JOURNAL OF APPLIED RESEARCH, vol. 3, no. 1, Apr. 2017, pp. 121-34, https://www.ajaronline.com/index.php/AJAR/article/view/204.