Automating warm-up length estimation

There are two key issues in assuring the accuracy of estimates of performance obtained from a simulation model. The first is the removal of any initialisation bias, the second is ensuring that enough output data is produced to obtain an accurate estimate of performance. This paper is concerned with the first issue, and more specifically warm-up estimation. Our aim is to produce an automated procedure, for inclusion into commercial simulation software, for estimating the length of warm-up and hence removing initialisation bias from simulation output data. This paper describes the extensive literature search that was carried out in order to find and assess the various existing warm-up methods, the process of short-listing and testing of candidate methods. In particular it details the extensive testing of the warm-up MSER-5 method. © 2010 Operational Research Society Ltd. All rights reserved

The behavior of transient period of nonterminating simulations: an experimental analysis

Cataloged from PDF version of article.The design and control of many industrial and service systems require the analysts to account for uncertainty. Computer simulation is a frequently used technique for analyzing uncertain (or stochastic) systems. One disadvantage of simulation modeling is that simulation results are only estimates of model performance measures. Therefore, to obtain better estimates, the outputs of a simulation run should undergo a careful statistical analysis. Simulation studies can be classified as terminating and nonterminating according to the output analysis techniques used. One of the major problems in the output analysis of nonterminating simulations is the problem of initial transient. This problem arises due to initializing simulation runs in an unrepresentative state of the steady-state conditions. Many techniques have been proposed in the literature to deal with the problem of initial transient. However, existing studies try to improve the efficiency and effectiveness of currently proposed techniques. No research has been encountered that analyzes the behavior of the transient period. In this thesis, we investigate the factors affecting the length of the transient period for nonterminating manufacturing simulations, particularly for serial production lines and job-shop production systems. Factors such as variability of processing times, system size, existence of bottleneck, reliability of system, system load level, and buffer capacity are investigated.Sandıkçı, BurhaneddinM.S

Modelling trauma hip fracture hospital activities

Hip fracture is the most common reason for an elderly person to be admitted to an acute orthopaedic ward. The main aim of this research is to provide a statistical evaluation of a hip fracture database, and then to use Operational Research (OR) techniques, using the statistical output, to model activities associated with the care of hip fracture patients. OR techniques employed in this thesis include simulation and queuing theory. This research focuses on hip fracture admissions to the University Hospital of Wales in Cardiff, with a primary aim of ascertaining whether the time between admission and surgical intervention has any impact upon patient outcome. Outcome is considered in terms of mortality, hospital length of stay and discharge destination. Statistical analyses are performed, via regression and CART analysis, to investigate length of stay and mortality variables. The results from these statistical tests are compiled, compared and investigated in more depth. Additionally, a principal component analysis is performed to investigate whether it would be feasible to reduce the dimensionality of the dataset, and subsequently principal component regression methodology is used to complement the output. Simulation is used to model activities in both the hip fracture ward and the trauma theatre. These models incorporate output from the statistical analysis and encompass complexities within the patient group and theatre process. The models are then used to test a number of ‘what-if’ type scenarios, including the future anticipated increase in demand. Finally, results from queuing theory are applied to the trauma theatre in order to determine a desired daily theatre allocation for these patients. Specifically, the M | G | 1 queuing system and results from queues with vacations are utilised. The thesis concludes with some discussion of how this research could be further expanded. In particular, two areas are considered; risk scoring systems and the Fenton-Wilkinson approximation