This paper addresses the problems associated with process performance measures, such as Cp. Cpk, when the frequency distribution of the variable being evaluation is not Normal. These measures, also known as capability indices, are commonly used in industry, yet they may not reflect the true process performance if the process distribution is not Normal. Gunter (1), in his four-part series articles, emphasized this point and other problems associated with measures like Cpk. In this paper, we discuss various scenarios with respect to process stability and frequency distribution, and provide an example using non-Normal process curve.
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Holmes, D. S. and Mergen, A. E. (1992). "Process Performance Measures When PRocess Distribution is Non-Normal," Istatistik, Journal of the Turkish Statistical Association, Vol. 2, No. 3, pp. 165-174.
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