A modified control chart for monitoring non-normal characteristics

Abstract

A process in control and capable of meeting customer requirements can enhance the survival and growth of the business. The systematic use of process improvement methodologies like lean, six sigma, etc., can improve the capability of the process significantly. When the process is highly capable, sometimes it may not make business sense to monitor the process using traditional control charts. It may be better to allow the process mean to vary over a small interval provided the fraction non-conforming is within a pre-specified value. This can be achieved by monitoring the process using the modified control chart. Most of the research on modified control chart discuss the case of normally distributed characteristics. In this paper, the authors extend the modified control chart methodology to monitor non-normal characteristics. The approach is based on Johnson's system of distributions. The suggested methodology is also demonstrated using two case studies in the paper.