Metrics and Conversions for Process Quality

In the wake of the many changes that business and industry has gone through during the past 20 years, one thing holds as sure as ever - measurement and data. That processes are best managed by taking appropriate data, by using clear metrics, and by scientific analysis is the very core of the way things are now done. In this paper we examine several of the mainstream metrics for process quality that have emerged as a standard by which quality is judged. We look at Normal distribution based metrics such as the indices Pp, Ppk, Cp and Cpk; the general fraction or percent non-conforming metric, defects per million opportunities; and the new “6-sigma” process capability standard. We consider the relations among these metrics and some aspects of the use of statistical confidence with respect to these metrics.