The Determination and Indetermination of Service Times in Manufacturing Systems

The notion of service times is of such fundamental importance in the analysis of queues that it has long been taken for granted. Intuitively, it is used to represent the time interval that a server is capable of completing a dispatched job. However, actual measurements of service times under simple queues in production lines have encountered practical difficulties, in spite of its seemingly deterministic nature. Previous studies have introduced concepts of effective process times to quantify service times. Besides notions of theoretical processing times, raw process times and queueing times, among others, are commonly used in various applications. Their existence causes confusion in the determination of service times and clarification of such terminologies is needed. A simple model is examined to quantify the various concepts and establish their interrelationships. This paper brings out new properties of effective process times with a dynamic dependence on utilization. Discrete event simulations are conducted to verify these properties and explain the phenomenon of indetermination of service times. Both theoretical prediction and simulation results show that unless the system is fully loaded, service time and effective process time are not equivalent and it cannot be measured directly from observations of effective process times.