Armann Ingolfsson

University of Alberta | UAlberta · Department of Accounting, Operations and Information Systems

Forecasting emergency medical service (EMS) call volumes is critical for resource allocation and planning. The development of many commercial and free software packages has made a variety of forecasting methods accessible. Practitioners, however, are left with little guidance on selecting the most appropriate method for their needs. Using 5 years o...

Preface to the Special Issue on Behavioral Queueing Science: The Need for a Multidisciplinary Approach Modern service systems are economically important but operationally complex. In “Preface to the Special Issue on Behavioral Queueing Science: The Need for a Multidisciplinary Approach,” Ingolfsson, Mandelbaum, Schultz, and Yom-Tov discuss how this...

We consider a Markovian multiserver queueing system with two customer classes, preemptive priorities, and reneging. We formulate this system as an infinite level-dependent quasi-birth-death process (LDQBD). We introduce an algorithm that endogenously truncates the level and calculates lower and upper bounds on stationary probabilities of this LDQBD...

Forecasting emergency medical service (EMS) call volumes is critical for resource allocation and planning. The development of many commercial and free software packages has made a variety of forecasting methods accessible. Practitioners, however, are left with little guidance on selecting the most appropriate method for their needs. Using 5 years o...

Importance Delays in transfer for discharge-ready patients from the intensive care unit (ICU) are increasingly described and contribute to strained capacity. Objective To describe the epidemiological features and health care costs attributable to potentially avoidable delays in ICU discharge in a large integrated health care system. Design, Setti...

Emergency systems are designed to almost always have enough capacity to respond to emergencies. However, capacity shortage periods do occur and these systems need to recover quickly. In this paper, we apply queueing models and study whether it is better for an emergency system to add or to expedite servers, in order to quickly recover from a capaci...

In this paper, we develop a general framework to analyze the influence of system load on service times in queueing systems. Our framework unifies previous results and ties them to possible future studies to help empirical and analytical researchers to investigate and model the ways in which load impacts service times. We identify three load charact...

Background: Strained intensive care unit (ICU) capacity represents a fundamental supply-demand mismatch in ICU resources. Strain is likely to be influenced by a range of factors; however, there has been no systematic evaluation of the spectrum of measures that may indicate strain on ICU capacity. Methods: We performed a systematic review to iden...

Servers in many real queueing systems do not work at a constant speed. They adapt to the system state by speeding up when the system is highly loaded or slowing down when load has been high for an extended time period. Their speed can also be constrained by other factors, such as geography or a downstream blockage. We develop a state-dependent queu...

Many service systems use case managers, servers who are assigned multiple customers and have frequent, repeated interactions with each customer until the customer’s service is completed. Examples may be found in healthcare (emergency department physicians), contact centers (agents handling multiple online chats simultaneously) and social welfare ag...

Background: The matching of critical care service supply with demand is fundamental for the efficient delivery of advanced life support to patients in urgent need. Mismatch in this supply/demand relationship contributes to "intensive care unit (ICU) capacity strain," defined as a time-varying disruption in the ability of an ICU to provide well-tim...

In this paper, we develop a general framework to analyze the influence of system load on service times in queueing systems. Our proposed framework unifies previous results and ties them to possible future studies to help empirical and analytical researchers to investigate and model the ways in which load impacts service times. We identify three loa...

Servers in many real queueing systems do not work at a constant speed. They adapt to the system state by speeding up when the system is highly loaded or slowing down when load has been high for an extended time period. Their speed can also be constrained by other factors, such as geography or a downstream blockage. We develop a state-dependent queu...

In this paper, we develop a general framework to analyze the influence of system load on service times in queueing systems. Our proposed framework unifies previous results and ties them to possible future studies to help empirical and analytical researchers to investigate and model the ways in which load impacts service times. We identify three loa...

We develop a general framework to help both empirical and analytical researchers to investigate and model how load impacts service times. We identify three load characteristics: "changeover," "instantaneous load," and "extended load." The load characteristics induce behaviors, or "mechanisms," in at least one of the system components: the "server,"...

We present models of trucks and shovels in oil sand surface mines. The models are formulated to minimize the number of trucks for a given set of shovels, subject to throughput and ore grade constraints. We quantify and validate the nonlinear relation between a shovel’s idle probability (which determines the shovel’s productivity) and the number of...

In this chapter I survey research on planning and management for emergency medical services, emphasizing four topics: forecasting demand, response times, and workload; measuring performance; choosing station locations; and allocating ambulances to stations, based on predictable and unpredictable changes in demand and travel times. I focus on empiri...

We propose and analyze a two-dimensional Markov chain model of an Emergency Medical Services system that repositions ambulances using a compliance table policy, which is commonly used in practice. The model is solved via a fixed-point iteration. We validate the model against a detailed simulation model. We demonstrate that the model provides accura...

Motivated by the dispatching of trucks to shovels in surface mines, we study optimal routing in a Markovian finite-source, multi-server queueing system with heterogeneous servers, each with a separate queue. We formulate the problem of routing customers to servers to maximize the system throughput as a Markov Decision Process. When the servers are...

We present an efficient, reliable, and easy-to-implement algorithm to compute steady-state probabilities for birth-death processes whose upper-tail probabilities decay geometrically or faster. The algorithm can provide any required accuracy and it avoids over-and underflow. In addition to steady-state probabilities, the algorithm can compute any pe...

This paper proposes a new approach, entitled discrete event simulation with probabilistic event list (DESPEL), for approximating the project completion time and the critical path in a stochastic resource–constrained project network. Unlike the traditional discrete event simulation approach, which is based on replications to find the project complet...

Staffing requirements for service systems are often determined by segmenting time into periods and using a sequence of steady-state queueing models. The resulting requirements are approximate because nonstationary and transient effects are not considered. We propose using a non-stationary infinite-server model to determine staffing requirements for...

This paper proposes new location models for emergency medical service stations. The models are generated by incorporating a survival function into existing covering models. A survival function is a monotonically decreasing function of the response time of an EMS vehicle to a patient that returns the probability of survival for the patient. The surv...

Like many others, the St. Louis Veterans Administration Medical Center (VAMC) Pharmacy help desk receives far more calls than can be processed by current staffing levels. The objective of the study is to improve pharmaceutical services provided by the call center, by using queueing theory and discrete event dynamic simulation to analyze incoming te...

This paper addresses the problem of scheduling ambulance crews in order to maximize the coverage throughout a planning horizon. The problem includes the subproblem of locating ambulances to maximize expected coverage with probabilistic response times, for which a tabu search algorithm is developed. The proposed tabu search algorithm is empirically...

Using administrative data for high-priority calls in Calgary, Alberta, we estimate how ambulance travel times depend on distance. We find that a logarithmic transformation produces symmetric travel-time distributions with heavier tails than those of a normal distribution. Guided by nonparametric estimates of the median and coefficient of variation,...

We describe a method to find low cost shift schedules with a time-varying service level that is always above a specified minimum. Most previous approaches used a two-step procedure: (1) determine staffing requirements and (2) find a minimum cost schedule that provides the required staffing in every period. Approximations in the first step sometimes...

I describe the game of ice hockey and the use of data analysis and mathematical modeling to answer questions of interest to ice hockey fans, coaches, and managers. Most of the discussion is in the context of the National Hockey League (NHL), the primary professional ice hockey league. I pay particular attention to two questions: when to pull the go...

We consider two capacity choice scenarios for the optimal location of facilities with fixed servers, stochastic demand, and congestion. Motivating applications include virtual call centers, consisting of geographically dispersed centers, walk-in health clinics, motor vehicle inspection stations, automobile emissions testing stations, and internal s...

This article categorizes existing maximum coverage optimization models for locating ambulances based on whether the models incorporate uncertainty about (1) ambulance availability and (2) response times. Data from Edmonton, Alberta, Canada are used to test five different models, using the approximate hypercube model to compare solution quality betw...

We describe an ambulance location optimization model that minimizes the number of ambulances needed to provide a specified service level. The model measures service level as the fraction of calls reached within a given time standard and considers response time to be composed of a random delay (prior to travel to the scene) plus a random travel time...

This article proposes new location models for emergency medical service stations. The models are generated by incorporating a survival function into existing covering models. A survival function is a monotonically decreasing function of the response time of an emergency medical service (EMS) vehicle to a patient that returns the probability of surv...

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1994. Includes bibliographical references (p. 209-213). by Ármann Ingólfsson. Ph.D.

Motivated by a Bell Canada call center operating in blend mode, we consider a system withtwo types of traffic and two types of agents. Outbound calls are served only by blend agents,whereas inbound calls can be served by either inbound-only or blend agents. Inbound callersmay balk or abandon. There are several performance measures of interest, includ...

We compare the performance of seven methods in computing or approximating service levels for nonstationary M(t)/M/s(t) queueing systems: an exact method (a Runge-Kutta ordinary-differential-equation solver), the randomization method, a closure (or surrogate-distribution) approximation, a direct infinite-server approximation, a modified-offered-load...

We outline the history, significance, and impact of four important contributions by Canadian researchers to stochastic modeling for operational research: the use of the uniformization method to compute transient probabilities for Markov chains, pioneered by Winfried K. Grassmann, contributions to Markov decision processes by Martin L. Puterman, con...

We develop and evaluate time-series models of call volume to the emergency medical service of a major Canadian city. Our objective is to offer simple and effective models that could be used for realistic simulation of the system and for forecasting daily and hourly call volumes. Notable features of the analyzed time series are: a positive trend, da...

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1991. Includes bibliographical references (leaves 115-118). by Ármann Ingólfsson. M.S.

The M/M/s queueing model assumes arrival rates, service rates, and number of servers to be constant in time. It is straightforward to extend the model to allow a time-varying arrival rate and a time-varying service rate, and the resulting models can be solved numerically using standard solvers for ordinary differential equations. The extension to a...

To calculate many of the important performance measures for an emergency response system, one requires knowledge of the probability that a particular server will respond to an incoming call at a particular location. Estimating these "dispatch probabilities" is complicated by four important characteristics of emergency service systems. We discuss th...

Several proposed path evaluation functions for hazardous materials transport use an approximation. Modified functions that avoid the approximation violate two reasonable axioms. We present new models by redefining the decision problem as one of satisfying demand at the destination. The new models satisfy the axioms and are relatively tractable.

We compare the performance of six methods in computing or approximating service levels for nonstationary M/M/s queueing systems: an exact method (a Runge Kutta ordinary differential equation solver), the randomization method, a closure (or surrogate distribution) approximation, a direct infinite server approximation, a modified offered load infinit...

Abstract Monte Carlo simulation,models,of regular season,and playoff games,in the National Hockey,League can be used,to motivate students,who,are sports fans to study OR/MS models. I describe how,such models,can be implemented in a spreadsheet, how my colleagues and I have used them in our courses, and how students have reacted to them. Editor's no...

We present a heuristic algorithm for preparing weekly employee schedules. The algorithm ensures that employees are not assigned to periods when they are unavailable or, more than one shift per day, or a night shift followed by a morning shift. It attempts as far as possible to satisfy constraints on the required number of employees per period, the...

The City of Edmonton's Emergency Medical Services (EMS) department proposed to move to a ‘single start station system’ (SS system) in which all ambulances would begin and end their shifts at the same location. We developed a discrete event simulation model to estimate the impact of this change and subsequently used this model to explore other chang...

this paper is to compare six proposed methods for calculating or approximating the service level, as defined above, for M(t)/M/s(t) queueing systems (with a nonstationary Poisson arrival process, exponentially distributed service times, and a time-varying number of servers) in terms of computational effort and accuracy

We present a heuristic algorithm for preparing weekly employee schedules. The algorithm ensures that employees are not assigned to periods when they are unavailable or, more than one shift per day, or a night shift followed by a morning shift. It attempts as far as possible to satisfy constraints on the required number of employees per period, the...

We developed a method for workforce scheduling that models both the structure of the set of permissible shifts, and the stochastic and time-varying demand process. A prototype implementation uses a genetic algorithm to search for good schedules, and evaluates the service level resulting from a schedule by numerically solving the equations of motion...

Graphical representations of spreadsheet queueing simulations can be used to teach students about queues and queueing processes. A customer graph shows the experience of every individual customer in a queue, based on arrival time, start of service, end of service, and showing clearly the length of time in queue and service time for each individual...

We offer an interactive spreadsheet to teach simple linear regression, which we have found to be an effective teaching tool. Unlike most textbooks that cover regression, this spreadsheet displays what is being minimized - the squares of the errors - and it allows students to experiment with the parameters of the line before optimizing them.

The most-widely used definition of risk in the hazardous materials transportation literature is the expected consequence of an incident (accident resulting in a release), which, for each edge of the network, is equal to the product of the incident probability and a quantifiable consequence (such as number of people evacuated). This definition ignor...

The run by run controller provides a framework for controlling a process which is subject to disturbances such as shifts and drifts as a normal part of its operation. The run by run controller combines the advantages of both statistical process control (SPC) and feedback control. It has three components: rapid mode, gradual mode, and generalized SP...

Revised October 1994 This paper examines the time lag between the peak in the arrival rate and the peaks in the mean and variance for the number of customers in an M(t)/M(t)/1l system. We establish a necessary condition for the time at which the peak in the mean is achieved. In cases in which system utilization exceeds one during some period, we sh...

The performance of a process control algorithm based on the EWMA statistic is analyzed. A simple condition relating the EWMA weight and the estimated process gain is shown to ensure that the control strategy is stable for a first-order multiple-input, single-output process that may be drifting or wandering. A similar condition is shown to guarantee...

The run-by-run (RbR) controller provides a framework for controlling the manufacturing process which is subject to disturbances such as shifts and drifts as a normal part of its operation. The RbR controller combines the advantages of statistical process control and feedback control to provide accuracy and flexibility of control that cannot be obta...

The Run by Run (RbR) Controller, which performs automated on-line optimization and control of VLSI processes, is considered. The RbR Controller combines statistical process control and feedback control to prescribe control responses which minimize the variation of the process output. It functions by adapting a model to the changing process and desi...

Summary form only given. The authors present an approach to process control in VLSI processing where the recipe is modified between every product run, based on measured results, in order to compensate for small drifts and shifts. This approach is implemented in the run-by-run controller, which is part of a process control system being developed at...

An approach to process control called generalized SPC which allows for the diagnosis of a process while the process is being tuned is discussed. A control module, the run by run controller, that implements a form of adaptive control based on the sequential design of experiments is discussed. Statistical process control is compared to the run by run...

Oil sands mining is costly, in large part due to the operation of trucks and shovels. We present models to minimize the number of trucks assigned to a set of shovels, subject to throughput and ore grade constraints. We quantify the nonlinear relation between a shovel's throughput and the number of trucks assigned to the shovel via an approximate ex...