A new process capability index for non-normal distributions

Jann Pygn Chen, Cherng G. Ding

Research output: Contribution to journalReview articlepeer-review

35 Scopus citations


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.

Original languageEnglish
Pages (from-to)762-770
Number of pages9
JournalInternational Journal of Quality and Reliability Management
Issue number7
StatePublished - 1 Dec 2001


  • Measurement
  • Process efficiency


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