Properties of the Cpm Index for Seemingly Unstable Production Processes

In this paper the use of the C pm index is extended to cover situations where the process mean, for example, due to shift-to-shift effects, varies on or around the target value. Approximate lower confidence intervals for the C pm index are derived for cases involving fixed and random factors and a simple relation between C pm and the minimum probability of conforming items is given. Finally, a sensitivity analysis is performed to examine the importance of the presence of a random process mean on the index and its lower confidence level.