9.1 Introduction

In this short Chapter we shall describe a number of new practical procedures for improving the analysis of process capability based on the indices C _P, C _pk, C _pm, S _pk and some others. Ever since Shewhart (1944) introduced his pioneering control charts, it has become a common practice for practitioners to use various control charts for monitoring different processes on a routine basis. As an example, when dealing with a variable data, the control chart technique usually employs a chart (such as a chart) to monitor the process center and a chart (such as an R chart or an S chart) to monitor the process spread. These charts are easy to comprehend, and they effectively communicate the critical process information without necessarily employing words and formulas. Unfortunately, they are applicable only for a single process (one process at a time). Thus, using them in multiprocess environment could be a clumsy and time-consuming task for supervisors or shop engineers since it may require to analyze each individual chart in order to evaluate the overall status of shop process control activities.

We have already pointed out that process capability index measures the ability of the process to reproduce products that meet certain specifications. When dealing with the capability of a process, there are mainly two characteristics of importance involved, the process location in relation to its target value and the process spread. The closer the process output is to the target...