Abstract
Process capability index Cpm, sometimes called Taguchi index, is a widely used measure to assess the ability of a process to cluster around the target. In calculating Cpm, it is assumed that the process data are independent, however, in many processes this assumption is violated. Because the exact distribution of estimator of Cpm for autocorrelated data is unknown, in this article, we derive explicit expressions for the moments of
and discuss its statistical properties in terms of these moments. The study shows that the estimator
is, in general, positively biased. Further, the exact values of mean and standard deviations of
are compared with the corresponding approximated values provided by earlier studies and it is found that the earlier studies overemphasize the effect of autocorrelation on
especially for smaller sample sizes, used for calculating
Acknowledgments
The authors would like to thank two anonymous reviewers and the Editor for their helpful and constructive comments that have significantly improved the manuscript.