An asymptotic confidence interval for the process capability index Cpm

The use of indices as an estimation tool of process capability is long-established among the statistical quality professionals. Numerous capability indices have been proposed in last few years. Cpm constitutes one of the most widely used capability indices and its estimation has attracted much interest. In this paper, we propose a new method for constructing an approximate confidence interval for the index Cpm. The proposed method is based on the asymptotic distribution of the index Cpm obtained by the Delta Method. Under some regularity conditions, the distribution of an estimator of the process capability index Cpm is asymptotically normal.