246 research outputs found

    'Some tactical problems in digital simulation' for the next 10 years

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    In his influential 1963 paper ‘Some Tactical Problems in Digital Simulation’, Conway identified important issues that became the pillars of research in simulation analysis methodology. Naturally these ‘problems’ were a product of the applications of interest at the time, as well as the state of simulation and computing, much of which has changed dramatically. In light of those changes, we attempt to identify the tactical problems that might occupy simulation researchers for the next 10 years

    A review of the open queueing network models of manufacturing systems

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    "December 1990."Includes bibliographical references (p. 52-58).Research partially supported by the "Leaders for Manufacturing Program". Research partially supported by the UCLA Senate Committee on Grants. 99by Gabriel R. Bitran, Sriram Dasu

    Error analysis of structured Markov chains

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    A Data-Driven Approach for Operational Improvement in Emergency Departments

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    Emergency departments (EDs) in the US are experiencing significant stress from crowding, of which one of the main contributors is the lengthy boarding process, which is the process of to-be-admit patients waiting in the ED for the hospital to ready beds for them. We explored ways to reduce crowding by initiating hospital bed request (BeRT) early on for likely to-be-admit patients. In Chapter 2, we modeled the ED patient flow as a Markov decision process. With the objective of balancing the tradeoff between waiting cost and the cost of false early BeRTs, we found the optimal early BeRT policy to be of threshold type, where the threshold is a function of census and patients probability of admission. Chapter 3 built a fluid model, where patients flow into the ED (a fluid tank) as continuous fluid flowing at a time-dependent deterministic rate. To control the number of false early BeRTs, we imposed a constraint on the length of time for the early BeRT option. The optimal policy that minimizes the fluid level (congestion level) in the ED dictates that when ED is under heavy traffic regime, one should BeRT early as early, and as long, as allowed. In chapter 4, we looked at several early BeRT heuristics that are inspired by the theoretical optimal policies found previously. We tested and compared their performances in terms of length-of-stay and waiting time using a simulation model built for the UNC ED based on 2012 patient data. We observed that as the admission probability distributions of the patient population became less variable, the heuristics that take more information into account performed better. Lastly, we offered a different perspective on ED crowding in Chapter 5, where we explored the association between ED cencus and providers’ triage and admission decisions. We found that the more crowded the ED was, the more conservative providers were, in that nurses tend to triage more patients as critical, and physicians tend to admit more patients into the hospital.Doctor of Philosoph

    Time-dependent stochastic methods for managing and scheduling Emergency Medical Services

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    Emergency Medical Services (EMS) are facing increasing pressures in many nations given that demands on the service are rising. This article focuses in particular on the operations of the Welsh Ambulance Service Trust (WAST), which is the only organisation that provides urgent paramedical care services on a day-to-day basis across the whole of Wales. In response to WAST’s aspiration to improve the quality of care it provides, this research investigates several interrelated advanced statistical and operational research (OR) methods, culminating in a suite of decision support tools to aid WAST with capacity planning issues. The developed techniques are integrated in a master workforce capacity planning tool that may be independently operated by WAST planners. By means of incorporating methods that seek to simultaneously better predict future demands, recommend minimum staffing requirements and generate low-cost rosters, the tool ultimately provides planners with an analytical base to effectively deploy resources. Whilst the tool is primarily developed for WAST, the generic nature of the methods considered means they could equally be applied to any service subject to demand that is of an urgent nature, cannot be backlogged, is heavily time-dependent and highly variabl

    Approximate solutions for multi-server queuing systems with Erlangian service times and an application to air traffic management

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.Includes bibliographical references (p. 209-213).This thesis is concerned with approximations of certain M(t)/G(t)/n(t)/n(t) + q queueing systems. More specifically, we are interested in such systems under very general conditions such as time-varying demand and capacity, and high utilization, including occasional oversaturation. Conditions such as these cannot be addressed with existing methodologies. We focus on M(t)/G(t)/n(t)/n(t) + q systems that can be approximated fairly well by M(t)/E&(t)/n(t)/n(t) + q systems. The latter have a large number of system states, that increase with the system parameters k, n, q and the utilization ratio, and involve complicated state transition probabilities. We propose numerical methods to solve the corresponding Chapman-Kolmogorov equations, exactly and approximately We first describe the exact solution technique of M(t)/Ek(t)/n(t)/n(t) + q queueing systems. Then, we develop two heuristic solution techniques of M(t)/E&(t)/ndt)/n(t) + q queueing systems, and provide the corresponding complete state descriptions. We compare the exact and approximate results to validate our heuristics and to select the heuristic that best approximates the exact results in steady-state and under stationary conditions. We also propose two algorithms to vary the number of servers in the system, since many real-life problems involve such changes in response to variations in demand. Further results using our ELC heuristic show that our practical approach behaves well under nonstationary conditions, including varying capacity, and during the transient period to steady-state. We conclude that our heuristic approach is an excellent alternative for studying and analyzing M(t)/E&(t)/n(t)/n(t)+q models and, as a by-product, many M(t)/G(t)/n(t)/n(t) +q systems that arise in practice. Finally, we present an application of the M(t)/E&(t)/n(t)/n(t) + q queueing model in the context of Air Traffic Management. This model appears to be a reasonable approach to estimating delays and congestion in an en-route sector in the air traffic system and can be used as an important building block in developing an analytical model of the entire Air Traffic Management system.by Marcos Escobar Fernández de la Vega.Ph.D

    Alternative methods of investigating the time dependent M/G/k queue

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    Thesis. 1976. M.S.--Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics.Microfiche copy available in Archives and Aero.Bibliograpy: leaf 154.by Peeter A. Kivestu.M.S

    Performance analysis at the crossroad of queueing theory and road traffic

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