process capability index is larger than 1. In this situation, it is
clear that the width of the specification limits is larger than the
width of the process variability. Therefore, a larger value of the
process capability index will increase the likelihood that the
process keeps a good proportion of conforming items in the
presence of small changes in the process or specification limits.
Many companies consider a minimum capability index fixed at
4=3, and some of them also have the aim of obtaining a value
of 5=3 for the process capability index.