• Journal of Korean Society for Quality Management
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• v.32 no.3
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• pp.109-125
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• 2004

This paper deals with an optimal process control problem in production of hull structural steel plate with high defective rate. The main quality characteristic(dependent variable) is the internal quality(defect) of plates and is dependent on process parameters(independent variables). The dependent variable(quality characteristics) has three categorical ordinal data and there are 35 independent variables(29 continuous variables and 6 categorical variables). In this paper, we determine the main factors and to develop the mathematical model between internal quality predicted probabilities and the main factors. Secondly, we find out the optimal process condition of main factors through analysis of variance(ANOVA) using simulation. We consider three models to obtain the main factors and the optimal process condition: linear, quadratic, error models.

• Industrial Engineering and Management Systems
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• v.12 no.1
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• pp.41-45
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• 2013

Age replacement policy is a commonly policy in maintenance management of spare part. It means that a spare part is always replaced at failure or fixed time after its installation, whichever occurs first. An optimal age replacement policy of spare parts concerns with finding the optimal replacement time determined by minimizing the expected cost per unit time. The age of the part was generally assumed to be a random variable in the past literatures, but in many situations, there are few or even no observed data to estimate the probability distribution of part's lifetime. In order to solve this phenomenon, a new uncertain age replacement policy has been proposed recently, in which the age of the part was assumed to be an uncertain variable. This paper discusses the optimal age replacement policies by dealing with the parts' lifetimes as different distributed uncertain variables. Several results on the optimal age replacement time are provided when the lifetimes are described by the uncertain linear, zigzag and lognormal distributions.

• Journal of Korean Society for Quality Management
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• v.16 no.2
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• pp.1-9
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• 1988

This paper considers a machine maintenance problem. The machine's condition is partially known by observing the machine's output products. This problem is formulated as an infinite horizon partially observable Markov decison process to find an optimal maintenance policy. However, even though the optimal policy of the model exists, finding the optimal policy is very time consuming. Thus, the intends of this study is to find ${\varepsilon}-optimal$ stationary policy minimizing the expected discounted total cost of the system, ${\varepsilon}-optimal$ policy is found by using a modified version of the well-known policy iteration algorithm. A numerical example is also shown.

• Journal of Korean Society for Quality Management
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• v.24 no.4
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• pp.44-58
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• 1996

This paper presents optimal accelerated life test plans with different censoring times for exponential, Weibull, and lognormal lifetime distributions, respectively. For an optimal plan, low stress level, proportion of test units allocated and censoring time at each stress are determined such that the asymptotic variance of the maximum likelihood estimator of a certain quantile at use condition is minimized. The proposed plans are compared with the corresponding optimal plans with a common censoring time over range of parameter values. Computational results indicate that those plans are statistically optimal ones in terms of accuracy of estimator when total censoring times of two plans are equal.

• Journal of the Korean Operations Research and Management Science Society
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• v.26 no.4
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• pp.83-97
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• 2001

Net Present Value (NPV) criterion has been the most widely used criterion to evaluate investment opportunities. However, the analysis based on the NPV criterion falls to consider the managerial flexibility of deferring decisions until major uncertainty is resolved. Recently, real options method attracted a lot of attention as a Powerful approach to address the problem. If investment decision is deferred, the value of the investment opportunity increases but opportunity cost increases at the same time. Therefore, it is important to decide the optimal timing how long the decision can be deferred. In this paper, we developed a model deciding the optimal decision timing. Using the real options approach, the model derived the optimal deferring time until a decision is made. Then, the model was applied to a Korean mobile telecommunications company who wants to invest on the wireless resale business. We believe that this model would be very useful to overcome the problem of NPV decision criterion. With this approach, we can make contingent decisions based on the observation of uncertainly resolutions.

• Journal of the Korean Operations Research and Management Science Society
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• v.28 no.4
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• pp.115-129
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• 2003

This paper suggests that the profit sharing contract can be Pareto optimal for both supplier and the purchaser. It is shown that Pareto optimal risk sharing contract can be obtained even though the decisions are made in a decentralized manner. The effect of risk attitude of the members of the supply chain is discussed. We examined various aspects of the risk sharing contract such as risk altitude, bargaining power, and cost of information system. The different risk attitude changes the optimal parameters and decision variables. Especially, we proved that, when both the supplier and the purchaser are risk averse, the purchaser orders less quantity than when the one is risk neutral and the other is risk averse. If the fixed cost for the information system is big enough to satisfy a certain condition, it is Pareto optimal not to share the profit and the purchaser takes all the risk even though he is risk averse.

• Journal of the Korean Operations Research and Management Science Society
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• v.36 no.1
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• pp.1-11
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• 2011

This article examines the impact of electronic commerce on environmental welfare. In particular, we analyze a game model of price competition between offline and online firms when consumption taxes are imposed on both offline and online transactions that produce environmental pollution. We investigate the properties of optimal taxation between offline and online markets and demonstrate that there is an optimal difference between the two taxes, depending upon not only the transaction cost between offline and online consumption, but also the environmental damage cost. We also investigate the effect of tax-free online transactions on tax revenues, and the financial feasibility of the optimal taxation.

• Journal of the Korean Operations Research and Management Science Society
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• v.6 no.1
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• pp.5-13
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• 1981

This paper considers a facility moving along a route of straight line to serve several facilities in xy plane. A route is obtained which minimizes the objective function expressed in terms of average rectilinear distance between the service facility and facilities to he served. The forms of the objective functions can be classified into several cases according to the initial and end points of the routes. In some cases, as in the problem of optimal location of new facility, the cordinates of the optimal routes are median of the cordinates of facilities to be served. In others, the optimal routes are obtained after examining the values of objective functions on the boundaries of slopes and intercepts. An overall optimal route is obtained by comparing these routes. The entire procedure has been programmed in BASIC language, and several case examples are given.

• Journal of Korean Society for Quality Management
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• v.28 no.2
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• pp.135-146
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• 2000

This study has adapted a genetic algorithm for an optimal design for the first time. The models using a simulation are the nonlinear models. Using an genetic algorithm in D-optimal, it is more efficient than previous algorithms to get an object function. Not like other algorithms, without any troublesome restrictions about the initial solution, not falling into a local optimal solution, it's the most suitable algorithm. Also if we use it without any adding experiments, we can use it to find optimal design of experimental condition efficiently.

• Journal of Korea Society of Digital Industry and Information Management
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• v.6 no.2
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• pp.183-191
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• 2010

Decision problem called an optimal release policies, after testing a software system in development phase and transfer it to the user, is studied. The applied model of release time exploited infinite non-homogeneous Poisson process. This infinite non-homogeneous Poisson process is a model which reflects the possibility of introducing new faults when correcting or modifying the software. The failure life-cycle distribution used mixture which has various intensity, if the system is complicated. Thus, software release policies which minimize a total average software cost of development and maintenance under the constraint of satisfying a software reliability requirement becomes an optimal release policies. In a numerical example, after trend test applied and estimated the parameters using maximum likelihood estimation of inter-failure time data, estimated software optimal release time.