N. Zahari, Z. Othman, N. A. Kedin, A. A. Aziz, and M. F. S. Bakhtiar, “Tipping Practices, Employee Motivation and Service Delivery Performance among 4 and 5-star Hotel Employees in Penang”, IMBR, vol. 17, no. 4(I), pp. 10-18, Nov. 2025.