SUMMARYThis paper proposes an economic model for the selection of time-varying control chart parameters for monitoring on-line the mean and variance of a normally distributed quality characteristic. The process is subject to two independent assignable causes. One cause changes the process mean and the other changes the process variance. The occurrence times of these assignable causes are described by Weibull distributions having increasing failure rates. The paper combines two existing models: (I) the model of Ohta and Rahim (IIE Transactions 1997; 29:481-486) for a dynamic economic design of X control charts, where a single assignable cause occurs according to a Weibull distribution and all design parameters are time varying; (II) the model of Costa and Rahim (QRE International 2000; 16:143-156) for the joint economic design of X and R control charts where two assignable causes occur independently according to Weibull distribution, with variable sampling intervals. The advantages of the proposed model over traditional X and R control charts with fixed parameters are presented.

This paper presents a method of integrating activity-based costing (ABC) and process simulation in human planning. Our studies have already proposed a method of integrating ABC and process simulation in business process reengineering (BPR) and showed a case study of a chemical plant. Some studies have also already showed some examples of various aspects in manufacturing systems. Although a large number of studies have been made on product-mix/machine loading or scheduling, little is known about human planning. In this paper, effective BPR methodologies to achieve dramatic improvements in business measures of workers' skills and costs based on ABC are discussed. First, two important tools: process simulation method and ABC analysis that can be customized by organizations for their own BPR are shown. As these tools have been separately used, a unified approach of process simulation and ABC analysis for process redesigns based on simulation. Secondly, a method of process simulation design is shown. It is repeated to consider working ratio and running time of resources. Thirdly, a method of ABC analysis that can be customized by organizations is shown. It automatically shows the data that has been gathered from many sources. By utilizing the data, the process simulation is implemented, and the result of simulation gives the data to ABC analysis. Lastly, this paper shows a case study in BPR and the effectiveness of our method.

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