المستخلص: |
This paper investigates some statistical issues of two related approaches to the evaluation of process performance, namely, Six Sigma and the capability indices (Cp and Cpk). The relationship between these approaches is clarified by exploring the role of Six Sigma in measuring (and predicting) the present (and future) performance of a process. Statistical techniques to reduce the sensitivity (lack of robustness) of capability indices to departures from normality are reviewed with reference to their credibility. In this context, three problematic inferential aspects are statistically investigated, in which an empirical lower bounds for the estimated indices cp and Cpk are derived. Suggestive recommendations are given as guide-lines in choosing the appropriate statistical technique for “future” analysis of the paper's open problems.
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