The C”_pk index for asymmetric tolerances: Implications and inference

Abstract.

The process capability index C_pk has been widely used in the manufacturing industry to provide numerical measures on process performance. Since C_pk is a yield-based index which is independent of the target T, it fails to account for process centering with symmetric tolerances, and presents an even greater problem with asymmetric tolerances. Pearn and Chen (1998) considered a new generalization C”_pk which was shown to be superior to other existing generalizations of C_pk for processes with asymmetric tolerances. In this paper, we investigate the relation between the fraction nonconforming and the value of C”_pk. Furthermore, we derive explicit forms of the cumulative distribution function and the probability density function for the natural estimator ”_pk, under the assumption of normality. We also develop a decision making rule based on the natural estimator ”_pk, which can be used to test whether the process is capable or not.