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A new process capability index for non‐normal distributions

Jann‐Pygn Chen (National Chin‐Yi Institute of Technology, Taichung, Taiwan, ROC, and)
Cherng G. Ding (National Chiao‐Tung University, Taipei, Taiwan, ROC)

International Journal of Quality & Reliability Management

ISSN: 0265-671X

Article publication date: 1 October 2001



Many process capability indices have been proposed to measure process performance. In this paper, we first review Cp, Cpk, Cpm and Cpmk, and their generalizations, CNp, CNpk, CNpm and CNpmk, and then propose a new index Spmk for any underlying distribution, which takes into account process variability, departure of the process mean from the target value, and proportion of nonconformity. Proportion of nonconformity can be exactly reflected by Spmk. Its superiority over CNpmk, a recently developed index, also taking into account process variability and departure from the target value, is demonstrated with several non‐normal processes. A method is proposed to estimate Spmk, with illustrations.



Chen, J. and Ding, C.G. (2001), "A new process capability index for non‐normal distributions", International Journal of Quality & Reliability Management, Vol. 18 No. 7, pp. 762-770.




Copyright © 2001, MCB UP Limited

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