The use of the C pm index to monitor processes

The C pm index was originally created to measure the ability of the process to produce products meeting specifications but, more recently, the C pm has also been used to control processes. In this new application, the charting points of the control chart are the estimated values of the C pm index obtained with the mean and the variance of the samples. The performance of the C pm chart is not affect by the values of the specification limits, but it is highly affected by d , the distance between the middle point of the specifications and the in‐control mean of the X distribution. Because of that, a search is conducted to find d that optimizes the overall performance of the C pm chart in signaling the three usual types of changes in process parameters: isolated mean shifts, isolated variance increases, and the two together. Comparing the joint and S 2 charts, the C pk chart and the C pm chart, the last one is always the most sensitive in detecting all three types of process changes, except when the variance remains unaltered and the mean increases/decreases a lot (one and half standard deviations); in this case, the joint and S 2 charts are slightly faster than the C pm chart.