A random interval estimation of the estimated process accuracy index

Abstract

Process accuracy index Ca has been proposed in the manufacturing industry to provide a numerical measure to assess process performance with respect to accuracy. Information regarding process centering (the ability to cluster around the specification mid-point) in existing engineering statistics and quality assurance literature is relatively rare. In this paper, a natural estimator of the accuracy index is considered under the normality assumption. The exact probability density function and the rth moment raised from a non-central chi-squared distribution of the estimated process accuracy index are derived. A decision-making procedure based on a random interval estimation of the considered index is also constructed. A practical example is demonstrated to illustrate how the proposed reliable procedure may be applied for in-plant applications.