WSC 2007 Final Abstracts

Analysis Methodology A Track

Monday 10:30:00 AM 12:00:00 PM
Ranking and Selection Procedures

Chair: Seong-Hee Kim (Georgia Institute of Technology)

Controlled Sequential Bifurcation for Software Reliability Study
Jun Xu and Feng Yang (West Virginia University) and Hong Wan (Purdue University)

Computer simulation is an appealing approach for the reliability analysis of structure-based software systems as it can accommodate important complexities present in realistic systems. When the system is complicated, a screening experiment to quickly identify important factors (components) can significantly improve the efficiency of analysis. The challenge is to guarantee the correctness of the screening results with stochastic simulation responses. Control Sequential Bifurcation (CSB) is a new screening methods for simulation experiments. With appropriate hypothesis testing procedures, CSB can simultaneously control the Type I error and power. The past research, however, has focused on responses with normal distributions. This paper proposes a CSB procedure with binary responses for software reliability analysis. A conditional sequential test for equality of two proportions is implemented to guarantee the the desired error control.

New Greedy Myopic and Existing Asymptotic Sequential Selection Procedures: Preliminary Empirical Results
Stephen E. Chick (INSEAD) and Juergen Branke and Christian Schmidt (Institute AIFB)

Statistical selection procedures can identify the best of a finite set of alternatives, where "best" is defined in terms of the unknown expected value of each alternative's simulation output. One effective Bayesian approach allocates samples sequentially to maximize an approximation to the expected value of information (EVI) from those samples. That existing approach uses both asymptotic and probabilistic approximations. This paper presents new EVI sampling allocations that avoid most of those approximations, but that entail sequential myopic sampling from a single alternative per stage of sampling. We compare the new and old approaches empirically. In some scenarios (a small, fixed total number of samples, few systems to be compared), the new greedy myopic procedures are better than the original asymptotic variants. In other scenarios (with adaptive stopping rules, medium or large number of systems, high required probability of correct selection), the original asymptotic allocations perform better.

A Tournament Framework for the Ranking and Selection Problem
Enver Yucesan (INSEAD)

A tournament can be broadly defined as a procedure that ranks agents, where they exhibit their performance in a noisy environment. By observing the agents' performance, the organizer computes their ranking and rewards them according to the revealed ranking. The organizerís challenge is therefore to determine the optimal tournament format that identifies the best agent in the most effective fashion. Tournaments thus provide a natural framework for ranking and selection (R&S) via simulation, which represents a set of approaches developed to complement the modeling flexibility of simulation with the efficiency of statistical techniques for effective decision making. In this paper, following the introduction of a general framework to represent various tournament formats and to assess their predictive power, we will report preliminary experimental results on the effectiveness of tournaments in identifying the best simulated system with the desired probability of correct selection in the presence of costs.

Monday 1:30:00 PM 3:00:00 PM
Output Analysis

Chair: Hernan Awad (University of Miami)

Low Bias Integrated Path Estimators
James Michael Calvin (New Jersey Institute of Technology)

We consider the problem of estimating the time-average variance constant for a stationary process. A previous paper described an approach based on multiple integrations of the simulation output path, and described the efficiency improvement that can result compared with the method of batch means (which is a special case of the method). In this paper we describe versions of the method that have low bias for moderate simulation run lengths. The method constructs an estimator based on applying a quadratic function to the simulation output. The particular quadratic form is chosen to minimize variance subject to constraints on the order of the bias. Estimators that are first-order and second-order unbiased are described.

Replicated Batch Means for Steady-State Simulations with Initial Transients
Christos Alexopoulos and Sigrun Andradottir (Georgia Institute of Technology), Nilay Tanik Argon (University of North Carolina at Chapel Hill) and David Goldsman (Georgia Institute of Technology)

We provide asymptotic expressions for the expected value and variance of the replicated batch means variance estimator when the stochastic process being simulated has an additive initial transient. These expressions explicitly show how the initial transient and the autocorrelation in the data affect the performance of the estimator. We apply our results to study how many replications will minimize the asymptotic bias of the variance estimator for a simple example.

Finite-sample Performance Guarantees for One-dimensional Stochastic Root Finding
Samuel M. T. Ehrlichman and Shane G. Henderson (Cornell University)

We study the one-dimensional root finding problem for increasing convex functions. We give gradient-free algorithms for both exact and inexact (stochastic) function evaluations. For the stochastic case, we supply a probabilistic convergence guarantee in the spirit of selection-of-the-best methods. A worst-case bound on the work performed by the algorithm shows an improvement over naÔve stochastic bisection.

Monday 3:30:00 PM 5:00:00 PM
Experimental Designs for Simulation

Chair: Hong Wan (Purdue University)

Metamodeling for Cycle Time-throughput-product Mix Surfaces Using Progressive Model Fitting
Feng Yang and Jingang Liu (West Virginia University) and Mustafa Tongarlak, Bruce E. Ankenman, and Barry L. Nelson (Northwestern University)

A simulation-based methodology is proposed to map the mean of steady-state cycle time as a function of throughput and product mix for manufacturing systems. Nonlinear regression models motivated by queueing analysis are assumed for the underlying response surface. To insure efficiency and control estimation error, simulation experiments are built up sequentially using a multistage procedure to collect data for the fitting of the models. The resulting response surface is able to provide a cycle-time estimate for any throughput and any product mix, and thus allows the decision maker to instantly investigate options and trade offs regarding their production planning.

Subset Selection and Optimization for Selecting Binomial Systems Applied to Supersaturated Design Generation
Ning Zheng and Theodore Allen (The Ohio State University)

The problem of finding the binomial population with the highest success probability is considered when the number of binomial populations is large. A new rigorous indifference zone subset selection procedure for binomial populations is proposed with the proof of the corresponding least favorable configuration. For cases involving large numbers of binomial populations, a simulation optimization method combining the proposed subset selection procedure with an elitist Genetic Algorithm (GA) is proposed to find the highest-mean solution. Convergence of the proposed GA frame work are established under general assumptions. The problem of deriving supersaturated screening designs is described and used to illustrate the application of all methods. Computational comparisons are also presented for the problem of generating supersaturated experimental designs.

Determining Efficient Simulation Run Lengths for Real Time Decision Making
Russell C. H. Cheng (University of Southampton)

Suppose that there are a number of alternative ways of operating a system, and a performance measure is available for comparing them. Simulation runs can be carried out to estimate this measure for different alternatives, but there are too many for all to be examined because there is a strict limit to the time available for simulations. If the accuracy with which the performance measure can be estimated increases with run length, then a balance has to be made between making few runs where the performance measure is accurately estimated and making a large number of runs but with the performance measure poorly estimated. We analyse how the best run length can be selected to ensure that an alternative is found with a good performance measure. This problem can arise in real time decision making, and we give a real example arising in the provision of fire service emergency cover.

Tuesday 8:30:00 AM 10:00:00 AM
Simulation Optimization

Chair: Jeff Hong (Hong Kong University of Science and Technology)

Stochastic Trust Region Gradient-free Method (STRONG)- A New Response-Surface-Based Algorithm in Simulation Optimization
Kuo-Hao Chang (Purdue University), L. Jeff Hong (Hong Kong University of Science and Technology) and Hong Wan (Purdue University)

Response Surface Methodology (RSM) is a metamodel-based optimization method. Its strategy is to explore small subregions of the parameter space in succession instead of attempting to explore the entire parameter space directly. This method has been widely used in simulation optimization. However, RSM has two significant shortcomings: Firstly, it is not automated. Human involvements are usually required in the search process. Secondly, RSM is heuristic without convergence guarantee. This paper proposes Stochastic Trust Region Gradient-Free Method (STRONG) for simulation optimization with continuous decision variables to solve these two problems. STRONG combines the traditional RSM framework with the trust region method for deterministic optimization to achieve convergence property and eliminate the requirement of human involvement. Combined with appropriate experimental designs and specifically efficient screening experiments, STRONG has the potential of solving high-dimensional problems efficiently.

Kriging Metamodeling in Constrained Simulation Optimization: An Explorative Study
William E. Biles (University of Louisville), Jack P. C. Kleijnen and Wim C. M. van Beers (Tilburg University) and Inneke van Nieuwenhuyse (European University College)

This paper describes two experiments exploring the potential of Kriging for constrained simulation optimization. Both experiments study an (s, S) inventory system, where the objective is to find the optimal values of s and S. The objective function and constraints used in these two experiments differ, as does the approach used to determine the optimum combination predicted by the Kriging model. The results of these experiments indicate that the Kriging methodology offers opportunities for solving constrained optimization problems in simulation; future research will focus on further refining the optimization procedure.

Allocation of Simulation Runs for Simulation Optimization
Alireza Kabirian and Sigurdur Olafsson (Iowa State University)

Simulation optimization (SO) is the process of finding the optimum design of a system whose performance measure(s) are estimated via simulation. We propose some ideas to improve overall efficiency of the available SO methods and develop a new approach that primarily deals with continuous two dimensional problems with bounded feasible region. Our search based method, called Adaptive Partitioning Search (APS), uses a neural network as meta-model and combines various exploitation strategies to locate the optimum. Our numerical results show that in terms of the number of evaluations (simulation runs) needed, the APS algorithm converges much faster to the optimum design than two well established methods used as benchmark.

Tuesday 10:30:00 AM 12:00:00 PM
Advances in Rare-Event Simulation Methodology

Chair: Jose Blanchet (Harvard University)

Importance Sampling of Compounding Processes
Jose H. Blanchet (Harvard University) and Bert Zwart (Georgia Institute of Technology)

Compounding processes, also known as perpetuities, play an important role in many applications; in particular, in time series analysis and mathematical finance. Apart from some special cases, the distribution of a perpetuity is hard to compute, and large deviations estimates sometimes involve complicated constants which depend on the complete distribution. Motivated by this, we propose provably efficient importance sampling algorithms which apply to qualitatively different cases, leading to light and heavy tails. Both algorithms have the non-standard feature of being state-dependent. In addition, in order to verify the efficiency, we apply recently developed techniques based on Lyapunov inequalities.

Path-sampling for State-dependent Importance Sampling
Jose H. Blanchet and Jingchen Liu (Harvard University)

State-dependent importance sampling (SDIS) has proved to be particularly useful in simulation (specially in rare event analysis of stochastic systems). One approach for designing SDIS is to mimic the zero-variance change-of-measure by using a likelihood ratio that is proportional to an asymptotic approximation that may be available for the problem at hand. Using such approximation poses the problem of computing the corresponding normalizing constants at each step. In this paper, we propose the use of path-sampling, which allows to estimate such normalizing constants in terms of one dimensional integrals. We apply path-sampling to estimate the tail of the delay in a G/G/1 queue with regularly varying input. We argue that such tail estimation can be performed with good relative precision in quadratic complexity (in terms of the tail parameter) - assuming that path-sampling is combined with an appropriate change-of-measure proposed by Blanchet and Glynn (2007a).

Efficient Suboptimal Rare-event Simulation
Xiaowei Zhang (Stanford University), Jose H. Blanchet (Harvard University) and Peter W. Glynn (Stanford University)

Much of the rare-event simulation literature is concerned with the development of asymptotically optimal algorithms. Because of the difficulties associated with applying these ideas to complex models, this paper focuses on sub-optimal procedures that can be shown to be much more efficient than conventional crude Monte Carlo. We provide two such examples, one based on ``repeated acceptance/rejection'' as a mean of computing tail probabilities for hitting time random variables and the other based on filtered conditional Monte Carlo.

Tuesday 1:30:00 PM 3:00:00 PM
Rare-Event Simulation

Chair: Pierre L'Ecuyer (University of Montreal)

Rare-event Simulation for a Multidimensional Random Walk with T-distributed Increments
Jose H. Blanchet and Jingchen Liu (Harvard University)

We consider the problem of efficient estimation via simulation of first passage time probabilities for a multidimensional random walk with t distributed increments. In addition of being a natural generalization of the problem of computing ruin probabilities in insurance - in which the focus is a one dimensional random walk - this problem captures important features of large deviations for multidimensional heavy-tailed processes (such as the role played by the mean of the random walk in connection to the spatial location of the target set). We develop a state-dependent importance sampling estimator for this class of multidimensional problems. Then, we argue - using techniques based on Lyapunov type inequalities - that our estimator is strongly efficient.

Estimating the Probability of a Rare Event Over a Finite Time Horizon
Pieter-Tjerk de Boer (University of Twente), Pierre L'Ecuyer (Universite de Montreal) and Gerardo Rubino and Bruno Tuffin (IRISA/INRIA Rennes)

We study an approximation for the zero-variance change of measure to estimate the probability of a rare event occuring when a continuous-time Markov chain reaches a given set of states before some fixed time limit. The jump rates of the chain are expressed as functions of a rarity parameter in a way that the probability of the rare event goes to zero when the rarity parameter goes to zero. After giving a general expression for the zero-variance change of measure in this situation, we develop an approximation of it via a power series and show that this approximation provides a bounded relative error when the rarity parameter goes to zero. We illustrate the performance of our approximation on numerical examples of highly reliable Markovian systems, and compare it to a previously proposed heuristic. We also exhibit the exact zero-variance change of measure and compare it with the approximations.

Ant-based Approach for Determining the Change of Measure in Importance Sampling
Poul Einar Heegaard (Norwegian University of Science and Technology) and Werner Sandmann (University of Bamberg)

Importance Sampling is a potentially powerful variance reduction technique to speed up simulations where the objective depends on the occurrence of rare events. However, it is crucial to find a change of the underlying probability measure yielding estimators with significantly reduced variance compared to direct estimators. In this paper, we present a new dynamic and adaptive method for this purpose. The method is inspired by ant-based systems that are in widespread use for solving optimization problems. No intimate knowledge of the model under consideration is necessary. Instead, the method adapts to it. Different commonly used modeling paradigms such as queueing and reliability models, amongst many others, are supported by describing the new method in terms of a transition class formalism. Simulation results demonstrate the accuracy of the obtained estimates, and details of the adapted change of measure are investigated to gain insights into the inner workings of the method.

Tuesday 3:30:00 PM 5:00:00 PM
Combining Simulation and Optimization

Chair: Tito Homem-de-Mello (Northwestern University)

Sequential Sampling for Solving Stochastic Programs
Guzin Bayraksan (The University of Arizona) and David Morton (The University of Texas at Austin)

We develop a sequential sampling procedure for solving a class of stochastic programs. A sequence of feasible solutions, with at least one optimal limit point, is given as input to our procedure. Our procedure estimates the optimality gap of a candidate solution from this sequence, and if that point estimate is sufficiently small then we stop. Otherwise, we repeat with the next candidate solution from the sequence with a larger sample size. We provide conditions under which this procedure: (i) terminates with probability one and (ii) terminates with a solution which has a small optimality gap with a prespecified probability.

Non-linear Control Variates for Regenerative Steady-State Simulation
Sujin Kim (Purdue University) and Shane G. Henderson (Cornell University)

We assume the existence of a parameterized family of control variates that could be used in a regenerative steady-state simulation. We show how such controls can be generated in the Markov-process setting, discuss the optimization problem of searching for a good choice of parameterization, and develop a strong law and central limit theorem for the resulting estimator.

Implications of Heavy Tails on Simulation-based Ordinal Optimization
Mark Broadie, Minsup Han, and Assaf Zeevi (Columbia University)

We consider the problem of selecting the best system using simulation-based ordinal optimization. This problem has been studied mostly in the context of light-tailed distributions, where both Gaussian-based heuristics and asymptotically optimal procedures have been proposed. The latter rely on detailed knowledge of the underlying distributions and give rise to an exponential decay of the probability of selecting the incorrect system. However, their implementation tends to be computationally intensive. In contrast, in the presence of heavy tails the probability of selecting the incorrect system only decays polynomially, but this is achieved using simple allocation schemes that rely on little information of the underlying distributions. These observations are illustrated via several numerical experiments and are seen to be consistent with asymptotic theory.

Wednesday 8:30:00 AM 10:00:00 AM
Advances in Simulation Output Analysis

Chair: Emily Lada (SAS Institute)

Confidence Interval Estimation Using Linear Combinations of Overlapping Variance Estimators
Tuba Aktaran-Kalayci (University at Buffalo (SUNY)), David Goldsman (Georgia Institute of Technology) and James R. Wilson (North Carolina State University)

We develop new confidence-interval estimators for the mean and variance parameter of a steady-state simulation output process. These confidence intervals are based on optimal linear combinations of overlapping estimators for the variance parameter. We present analytical and simulation-based results exemplifying the potential of this technique for improvements in accuracy for confidence intervals.

Folded Standardized Time Series Area Variance Estimators for Simulation
Claudia Antonini (Universidad Simon Bolivar), Christos Alexopoulos and David Goldsman (Georgia Institute of Technology) and James R. Wilson (North Carolina State University)

We estimate the variance parameter of a stationary simulation-generated process using ``folded'' versions of standardized time series area estimators. We formulate improved variance estimators based on the combination of multiple folding levels as well as the use of batching. The improved estimators preserve the asymptotic bias properties of their predecessors but have substantially lower variance. A Monte Carlo example demonstrates the efficacy of the new methodology.

Sbatch: A Spaced Batch Means Procedure for Simulation Analysis
Emily K. Lada (SAS Institute Inc.) and James R. Wilson (North Carolina State University)

We discuss SBatch, a simplified procedure for steady-state simulation analysis that is based on spaced batch means, incorporating many advantages of its predecessors ASAP3 and WASSP while avoiding many of their disadvantages. SBatch is a sequential procedure designed to produce a confidence-interval estimator for the steady-state mean response that satisfies user-specified precision and coverage-probability requirements. First SBatch determines a batch size and an interbatch spacer size such that beyond the initial spacer, the spaced batch means approximately form a stationary first-order autoregressive process whose lag-one correlation does not significantly exceed 0.8. Next SBatch delivers a correlation-adjusted confidence interval based on the sample variance and lag-one correlation of the spaced batch means as well as the grand mean of all the individual observations beyond the initial spacer. In an experimental evaluation, SBatch compared favorably with ASAP3 and WASSP.

Wednesday 10:30:00 AM 12:00:00 PM
Input Modeling

Chair: Michael Kuhl (Rochester Institute of Technology)

A Method for Fast Generation of Bivariate Poisson Random Vectors
Kaeyoung Shin and Raghu Pasupathy (Virginia Tech)

It is well known that trivariate reduction - a method to generate two dependent random variables from three independent random variables - can be used to generate Poisson random variables with specified marginal distributions and correlation structure. The method, however, works only for positive correlations. Moreover, the proportion of feasible positive correlations that can be generated through trivariate reduction deteriorates rapidly as the discrepancy between the means of the target marginal distributions increases. We present a specialized algorithm for generating Poisson random vectors, through appropriate modifications to trivariate reduction. The proposed algorithm covers the entire range of feasible correlations in two dimensions, and preliminary tests have demonstrated very fast preprocessing and generation times.

Classification Analysis for Simulation of Machine Breakdowns
Lanting Lu, Christine S. M. Currie, and Russell C. H. Cheng (University of Southampton) and John Ladbrook (Ford)

Machine failure is often an important factor in throughput of manufacturing systems. To simplify the inputs to the simulation model for complex machining and assembly lines, we have derived the Arrows classification method to group similar machines, where one model can be used to describe the breakdown times for all of the machines in the group and breakdown times of machines can be represented by finite mixture model distributions. The Two-Sample Cramer-von Mises statistic is used to measure the similarity of two sets of data. We evaluate the classification procedure by comparing the throughput of a simulation model when run with mixture models fitted to individual machine breakdown times; mixture models fitted to group breakdown times; and raw data. Details of the methods and results of the grouping processes will be presented, and will be demonstrated using an example.

Analysis and Generation of Random Vectors with Copulas
Christoph Strelen (University of Bonn) and Feras Nassaj (Fraunhofer Institute)

Copulas are used in finance and insurance for modeling stochastic dependency. They comprehend the entire dependence structure, not only the correlations. Here they are estimated from measured samples of random vectors. The copula and the marginal distributions of the vector elements define a multivariate distribution of the sample which can be used to generate random vectors with this distribution. This can be applied as well to time series. A programmed algorithm is proposed. It is fast and allows for random vectors with high dimension, for example 100.

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