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Essays on Queueing Systems with Endogenous Service Times Open Access


Other title
Behavioral operations
Quasi-birth-death processes
Queueing Systems
Endogenous Service Times
Type of item
Degree grantor
University of Alberta
Author or creator
Delasay Sorkhab, Mohammad
Supervisor and department
Ingolfsson, Armann (School of Business)
Kolfal, Bora (School of Business)
Examining committee member and department
Hagtvedt, Reidar (School of Business)
Faculty of Business
Operations and Information Systems
Date accepted
Graduation date
Doctor of Philosophy
Degree level
Service rates in many real queueing systems, e.g., call centers and emergency departments, change with the system conditions. We investigate and model load-dependant service rates in this dissertation. First, we propose a general framework that explains different mechanisms that cause service rates to change in response to the system load. We use the framework to categorize and explain the results of published empirical papers that document dependence of service times on load. We employ the framework to analyze the effect of load on service times of an Emergency Medical Services (EMS) system based on a data set for emergency calls received by the Calgary EMS system in 2009. Second, we propose a state-dependent queueing model in which servers speed up in response to the system “load,” but eventually slow down as a result of “overwork,” a situation where the system has been under a heavy load for an extended time period. We quantify load as the fraction of occupied servers and we operationalize overwork as the number of users served so far in the current high-load period. Our model is a quasi-birth-and-death process with a special structure that we exploit to develop efficient algorithms to compute system performance measures. We use the model and simulation to demonstrate how using models that ignore adaptive server behavior can result in inconsistencies between planned and realized performance and can lead to suboptimal, unstable, or oscillatory staffing decisions.
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