Browsing by Author "Shu, Ming-Hung"

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A comparison of methods for estimating loss-based capability index
(2009)
Wu, Chien-Wei
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Shu, Ming-Hung
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Pearn, W. L.
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Cheng, Feng-Tsung
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工業工程與管理學系
The process capability index Cpm, sometimes called the loss-based index, has been proposed to the manufacturing industry for measuring process reproduction capability. This index incorporates the variation of production items with respect to the target value and the specification limits preset in the factory. To estimate the loss-based index properly and accurately, certain frequentist and Bayesian perspectives have been proposed to obtain lower confidence bounds (LCBs) for providing minimum process capability. The LCBs not only provide critical information regarding process performance but are also used to determine whether an improvement was made in a capability index and by extension in reducing the fraction of non-conforming items. In this paper, under the assumption of normality, based on frequentist and Bayesian senses, several existing approaches for constructing LCBs of Cpm are presented. Depending on the statistical methods used, we then classify these existing approaches into three categories and compared them in terms of the coverage rates and the mean values of the LCBs via simulations. The relative advantages and disadvantages of these approaches are summarized with some highlights of the relevant findings.
Estimating and testing process accuracy with extension to asymmetric tolerances
(2010-08-01)
Wu, Chien-Wei
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Shu, Ming-Hung
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Pearn, W. L.
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Tai, Yi-Chang
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工業工程與管理學系
Pearn et al. (Commun. Stat. Theory Methods, 27(4):985-1000, 1998) introduced the process accuracy index C(a) to measure the degree of process centering, the ability to cluster around the center. In this paper, we derive an explicit form of the cumulative distribution function for the estimator (C) over cap (a) with the case of symmetric tolerances. Subsequently, the distributional and inferential properties of the estimated process accuracy index C(a) are provided. Calculations of the critical values, P-values, and lower confidence bounds are developed for testing process accuracy. Further, a generalization of C(a) for the case with asymmetric tolerances is proposed to measure the process accuracy. Based on the results practitioners can easily perform the testing of the process accuracy, and make reliable decisions on whether actions should be taken to improve the process quality. An application is given to illustrate how we test the process accuracy using the actual data collected from the factory.