ENBIS: European Network for Business and Industrial Statistics

Estimated Process Capability Indices and Impact on the Proportion of Nonconforming Output

Abstract

Submitted by

Murat Caner Testik

Authors

Murat Caner Testik and Banu Yuksel Ozkaya

Affiliation

Hacettepe University, Dept. of Industrial Engineering, Ankara TURKEY

Abstract

Process capability analyses play a vital role in quality improvement programmes. Among the process capability analysis tools, commonly used are the process capability indices (PCIs). These are single number assessments of the relationship between process performance and specifications on a quality characteristic. Nevertheless, process parameters are often unknown in practice and need to be estimated for use in PCIs. Hence, estimated PCIs are subject to sampling error. In this study, two widely used process capability indices, namely Cp and Cpk, are considered when the observations are normally distributed but the process mean and/or variance are estimated. Marginal distributions for the proportion of nonconforming items are developed and computed for several sample sizes. Some recommendations are provided for practitioners.

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