Process capability indices for a regularly adjusted process

Process capability indices are widely used in industry to achieve and maintain a high-quality level of manufactured items. The successful application of process capability indices requires the process to be stationary. This is not the case for trended processes with regular adjustment, which are very common in manufacturing processes. An interesting and important question is how to define and interpret the process capability for such a process. In this paper, the traditional process capability indices are modified for these trended processes. Two different sources of process variation, one from the process trend, the other from random variation, are identified and used to develop new process capability indices. Based on these new indices, we show that the capability of trended processes can be improved through periodic adjustment to its maximum achievable value. We also develop the procedures to determine the adjustment frequency to meet any prespecified requirements on the capability index or on the process yield.