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In this essay, we examine what-is, what-might-be, and what-should-be computational models where the purpose is to explore new concepts, ideas, boundaries, and limitations going beyond what we know at the [...]

A family-based association method, called the generalized disequilibrium test (GDT), is used to assess dichotomous genotype traits within a family. Computer simulations and data applications confirm that the GDT is advantageous compared to other existing methods since it incorporates information beyond first-degree relatives and family size for analysis, and is also equipped with a robust technique which is not based on large sample theory.

Understanding and reproducing complex human oculomotor behaviors using computational models is a challenging task. In this paper, two studies are presented, which focus on the development and evaluation of a computational model to show the influences of cyclic top-down and bottom-up processes on eye movements. To explain these processes, reinforcement learning was used to control eye movements. The first study showed that, in a picture-viewing task, different policies obtained from different picture-viewing conditions produced different types of eye movement patterns. In another visual search task, the second study illustrated that feedback information from each saccadic eye movement could be used to update the model's eye movement policy, generating different patterns in the following saccade. These two studies demonstrate the value of an integrated reinforcement learning model in explaining both top-down and bottom-up processes of eye movements within one computational model. Index Terms--Cognitive model, eye movement, queueing network model, reinforcement learning.

It is said that experience is the best teacher. We want to share some project experiences that have taught us important lessons over the years. Some projects are recent; others […]

Power-line communications (PLC) is a field that has raised a lot of research in the past years. In this paper, we introduce array codes into the PLC environment and we study the channel's performance. In particular, generalized array codes (GAC), as well as row and column array codes (RAC) are applied. We examine their performance by obtaining three different code rates from each category. Therefore, the (8, 4, 4), (12, 7, 4) and (16, 11, 4) GAC codes as well as the (9, 4, 4), (12, 6, 4), and (15, 8, 4) RAC codes are used, meaning that we obtain code rates of (1/2), (7/12), (11/16), and (4/9), (6/12), (8/15) respectively. In addition, for reasons of completeness, convolutional codes are also being applied under the same channel conditions. Moreover, we suggest a hybrid coding technique, which combines the (8, 4, 4) GAC and the (15, 8, 4) RAC code in order to meet the requirements of the PLC time-varying channel and improve its performance. Concerning the system's design, we take into consideration Zimmermann's model for the PLC channel. We apply Middleton's model for the channel's background and impulsive noise, while we also introduce a novel way of estimating the system's impulsive noise. Finally, the well-known transmission technique of orthogonal frequency-division multiplexing is used. The channel's performance is evaluated in terms of the bit-error rate for different [E.sub.b]/[N.sub.o] values via computer simulations. Index Terms--Generalized array codes (GACs), impulsive noise, power-line communications (PLC), row column codes (RAC codes).

More than five hundred librarians have been experimenting with providing library services within Second Life, a Multi User Virtual Environment (MUVE) with over thirty thousand users online at any time. [...]

Detailed analyses of concrete and buried concrete structures undergoing complex inelastic responses to loads, such as those resulting from explosive detonations, are challenging mechanics problems and can require significant computational resources. The writers have been involved in the development of various constitutive models that are successful in modeling blast responses, but can also be computationally intensive--thus excluding their use for many large-scale applications. Recent efforts at the U.S. Army Engineer Research and Development Center have focused on developing procedures for performing these types of analyses in a production setting utilizing high performance computing. These models have been implemented into a parallel finite-element code, ParaAble, developed by the writers, and a new feature was added to the METIS partitioning software to easily apply weighting for improved load balancing in multiple material problems. Examples are shown that efficiently utilize from dozens to up to thousands of processors, both single and dual core, on Linux clusters and other parallel systems to enable such analyses to be performed in a reasonable amount of time. CE Database subject headings: Computation; Concrete; Soils; Finite element method; Buried structures.

An evaluation of metamodels quality of fit, using computer simulations, is analyzed.

The London Handicap Scale method of data modeling, with respect to adult respondents fron Netherlands, is evaluated.

A system modelling approach for the progression of diabetic retinopathy is described that is used for cost effectiveness evaluations of various prevention and patient care options. The necessary modular development guided by clicians and health care modellers are mentioned.

Newly developed approaches for construction simulation have been used to model the one-plant-multisite ready mixed concrete (RMC) production system, validated by real-life operations data in Hong Kong. HKCONSIM--a computer system for simulation modeling and analysis of Hong Kong's RMC production operations was developed in-house. The system is suitable for the resource provision planning and the production planning of a RMC plant, so as to meet given demands at a number of sites for concrete over a working day. The emphasis of the simulation modeling is on the interactions of multiple sites with the plant, and the objective is to improve the supply service levels and the utilization of plant resources. One significant improvement achieved via the research is that to simulate the RMC production operations with HKCONSIM does not require familiarity by the user with any software-specific terminology and modeling schematics, in contrast with existing simulation methods; the process of constructing a simulation model is reduced to specifying the attributes for each pour and site and providing the plant and truckmixer resources available on self-explanatory on-screen forms. Therefore, by experimenting on a HKCONSIM produced model the user, being a practitioner in the concreting industry, can readily study complicated relationships between the pattern of demand for concrete, the resources available to the system, and the service levels achieved together with the utilization levels achieved for the resources involved. Conclusions are given on the research and recommendations for future work made. CE Database subject headings: Simulation; Concrete; Construction planning; Hong Kong; Manufacturing.

The methodology of discrete-event simulation provides a promising alternative solution to designing and analyzing dynamic, complicated, and interactive construction systems. In this paper, an attempt is made to extend the previous work of simplifying construction simulation by delving into the fundamental approaches for discrete-event simulation. A new simplified discrete-event simulation approach (SDESA) is presented through extracting the constructive features from the existing event/activity-based simulation methods; both the algorithm and the model structure of simulation are streamlined such that simulating construction systems is made as easy as applying the critical path method (CPM). Two applications based on real road construction projects in Hong Kong serve as case studies to illustrate the methodology of simulation modeling with SDESA and reveal the simplicity and effectiveness of SDESA in modeling complex construction systems and achieving the preset objectives of such modeling. They are a granular base-course construction system featuring both cyclic and linear processes and an asphalt paving construction system with complicated technological/logical constraints. As a general-purpose method for construction planning, SDESA enables practitioners to deal with what the CPM-based network analysis method fails to solve by offering discrete-event simulation capabilities. Furthermore, the SDESA can potentially be adapted to special-purpose simulation tools to tackle large and complicated construction systems of practical size that have yet to find convenient solutions with existing simulation methods. CE Database subject headings: Simulation; Construction planning; Computer models; Road construction.

This paper presents a new architecture layout for a real-time power system simulator based on a distributed cluster of IBM PC-compatible desktop computers. A real-time network simulator based on a PC cluster can successfully cope with the size requirements of growing power systems and the computational demands of fast transient studies. A powerful product has been developed using off-the-shelf Pentium II 400-MHz workstations with a commercial real-time operating system, standard input/output (I/O) interfaces, and a multimachine networking scheme. Models based on the standard tool for power systems transients simulations, the Electromagnetic Transients Programs (EMTP) program, optimized for real-time performance ensure accurate simulation results. Index Terms--EMTP, OVNI, PC-cluster, power system simulator, real time.

Weather simulation models are commonly used to generate synthetic daily weather for use in studies of crop growth, water quality, water availability, soil erosion, climate change, and so on. Synthetic weather sequences are needed if long-term measured data are not available, measured data contain missing records, collection of actual data is cost or time prohibitive, or when necessary to simulate impacts of future climate scenarios. Most weather generators are capable of producing one or more components of weather such as precipitation, temperature, solar radiation, humidity, and wind speed. This study focused on one generation component, the procedure commonly used by weather simulation models to generate daily maximum and minimum temperature. The normal distribution is used by most weather generators (including USCLIMATE, WXGEN, LARS-WG, CLIMGEN, and CLIGEN) to generate daily maximum and minimum temperature values. The objective of this study was to analyze the adequacy of generating temperature data from the normal distribution. To accomplish this objective, the assumption of normality in measured daily temperatures was evaluated by testing the hypothesis that daily minimum and maximum temperature are normally distributed for each month. In addition, synthetic temperature records generated with the normal distribution were compared with measured temperature records. Based on these analyses, it was determined that measured daily maximum and minimum temperature are generally not normally distributed in each month but often are slightly skewed, which contradicts the assumption of normality used by most weather generators. In addition, generating temperature from the normal distribution resulted in several physically improbable values.

Migration is a universal but poorly understood human behavior, and new analytic tools for studying migration are badly needed. In this article, I describe a new computer simulation method for migrating human populations, which moves simulated human 'actors' on a lattice of points according to simple probability rules. The method includes models of birth, death, crowding, and competition, as well as of migration. Simulations are developed for a number of specific problems, including migration into empty continents, the effect of competition and migration on the spatial distribution of populations, the effect of biased migration on urban population structure, and the distribution of city sizes determined by migration. Direct comparison of simulations with measured population distributions demonstrates the plausibility of the models. The simulations suggest that seemingly complex problems of human space-time population dynamics may be resolved into simple behavioral rules and that migration phenomena can be subjected to scientific analysis. [Key words: migration, computer simulation, random walk, urban structures]

This paper aims to evaluate computerized model-selection strategies and the Monte Carlo experiments show that an econometric model-selection process called PcGets (general to specific) recovers the data generating process specification.

This paper describes a new technique for modeling hurricane risk in the United States. A storm track modeling approach is employed where, for each hurricane, the entire track as it crosses the ocean and makes landfall is modeled. The central pressure is modeled as a function of the sea surface temperature. The approach is validated through comparisons of simulated and observed key hurricane statistics (central pressure, translation speed, heading, and approach distance) along the U.S. coastline. The simulated and observed landfall rates of intense hurricanes (Saffir-Simpson Scale 3 and higher) also are compared on a regional basis along the coast. The model is able to reproduce the continuously varying hurricane climatology along the U.S. coastline, and it provides a rational means for examining the hurricane risk for geographically distributed systems such as transmission lines and insurance portfolios.

A problem often arising in engineering applications of computer models is to determine the importance of each data item in the large pool of required input factors. This paper explores a statistical approach for investigating factor sensitivities. The methodology is demonstrated with the HDM-III highway life-cycle cost analysis model. Specifically, the net present value (NPV) of life-cycle costs predicted by the HDM-III model is analyzed, and sensitivities of NPV to the link characterization input factors are investigated. In the statistically designed experiment, combinations of the input factors are chosen using a method called Latin hypercube sampling, which is well suited to the deterministic HDM-III model. Two analyses of the output data are performed, based on a first-order regression approximation and a Gaussian stochastic-process model. For NPV, the factor rankings are similar, but the sensitivities obtained from the two techniques show some marked differences. This demonstrates the greater flexibility of the stochastic-process model in dealing with nonlinearities and factor interactions in complex input-output relationships.

Prediction is an important cognitive activity in any decision-making process. For dynamic process control tasks, this activity is crucial, but studying it in a real-life environment is difficult. This article describes how a computerized simulation was used to study prediction in a complex environment. Using the simulation, the effects of many factors on the accuracy of prediction we re successfully measured. These factors are mainly the nature of the task, the method of presentation of the information, the number of repetitions of a specific task, and the length of the time taken for prediction. Three different tasks and four distinct interfaces were used in the simulation. Prediction was found to be significantly affected by all of the factors except the number of repetitions. Surprisingly, no learning effect was observed throughout the accomplishment of the various tasks., KEYWORDS: computerized simulation; dynamic process control; information presentation; prediction; work complex environments. The control of processes in dynamic environments has been the subject of many studies and has been found [...]

This paper discusses different computer tools used to help deliver, administer, and teach the material covered in two basic undergraduate courses in electrical engineering. The computer facilities, programs, and tutorials developed are discussed; however, the paper concentrates more on analyzing the results of using these tools in the student learning process. The students' marks and the results of several student surveys, as well as the teaching staff's observations, are used to evaluate the usefulness of the different tools and determine their advantages and disadvantages. Some unexpected results regarding the actual students' use of these tools are also presented and thoroughly analyzed. Index Terms - Computer learning, computers in lectures, course administration, simulation programs, student response.

Simulation technology provides powerful tools for analysis of information system (IS) designs, but it is a fact that the purpose of such technology is to enhance the quality of decisions made by human beings. On-line help and knowledge-based support, graphics, and Visual Interactive Simulation (VIS) and animation are particularly promising technologies to enhance the effectiveness of simulation from a human factors perspective. Based on literature review and two studies undertaken by the authors, the role of these technologies in simulation-based decision making is considered The authors find that there are opportunities to improve the decision support role if simulation technology in IS with enhanced user interface technology but that there are further issues to address. Although there are many practical difficulties in experimental assessment of human factors in IS simulation, such research is essential in determining the relevance of simulation technology to decision support., KEYWORDS: animation; decision support, graphics; human-computer interaction, knowledge-based help; simulation environments; simulation of information systems. Few major decisions about information system (IS) design and implementation are made in a completely [...]

Water managers have been attempting to operate reservoirs in a 'best water management strategy' to meet specific project objectives since the first man-made impoundment was constructed. When it was realized, several years ago, that the U.S. Army Corps of Engineers must have the capability to analyze the operation of large multiple reservoir systems for water quality, the Hydrologic Engineering Center (HEC) was funded to develop a computer program to meet this need. 'HEC-5Q, Simulation of Flood Control and Conservation Systems' computer model has the unique capabilities to accept user-specified water quantity and quality needs systemwide, and to decide how to regulate a network of reservoirs. The decision criteria are programmed to consider flood control, hydropower, instream flow (municipal, industrial, irrigation, water supply, fish habitat), and water-quality requirements. A brief history and description of the HEC-5Q model is provided along with citations to related case studies and technical reports.

The conceptual business life cycle literature suggests that at some stage in an entity's life a period of significant growth will be experienced. The level of growth may be slow and at a relatively constant rate, say for example from startup until the business reaches a level of trade consistent with owner(s) requirements. Some business will however, experience periods of significant growth, which may be either planned or unplanned. It is the management of this growth and the problems that growth engenders which has been the basis for much of the conceptual literature, including the stage models of growth. The model presented in this paper extends the existing growth model literature by detailing the impact of fluctuating growth rates on firm solvency, which considers the liquidity constraining conditions under which growth firms operate. The results suggest that the impact of fluctuating growth rates should be imputed into growth models which, todate, have been based on the simplified assumption of constant growth, with limited consideration of the effect of boundary constraints., A Monte Carlo Simulation Analysis of the Effects of Volatility on the Rate of Sustainable Growth 1 Introduction A number of models have been detailed within the literature which outline [...]