Process capability assessment for index C pk based on bayesian approach

W.l. Pearn*, Chien Wei Wu

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


Process capability indices have been proposed in the manufacturing industry to provide numerical measures on process reproduction capability, which are effective tools for quality assurance and guidance for process improvement. In process capability analysis, the usual practice for testing capability indices from sample data are based on traditional distribution frequency approach. Bayesian statistical techniques are an alternative to the frequency approach. Shiau, Chiang and Hung (1999) applied Bayesian method to index C pm and the index C pk but under the restriction that the process mean μ equals to the midpoint of the two specification limits, m. We note that this restriction is a rather impractical assumption for most factory applications, since in this case C pk will reduce to C p . In this paper, we consider testing the most popular capability index C pk for general situation - no restriction on the process mean based on Bayesian approach. The results obtained are more general and practical for real applications. We derive the posterior probability, p, for which the process under investigation is capable and propose accordingly a Bayesian procedure for capability testing. To make this Bayesian procedure practical for in-plant applications, we tabulate the minimum values of Ĉ pk for which the posterior probability p reaches desirable confidence levels with various pre-specified capability levels.

Original languageEnglish
Pages (from-to)221-234
Number of pages14
Issue number2
StatePublished - 1 Apr 2005


  • Bayesian approach
  • Posterior distribution
  • Posterior probability
  • Process capability indices

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