Suppose that you have a process which achieves the Cp and Cpk values which your customer demands. Is this the end of the story? Can you sit back and relax, secure in the knowledge that you will be more than able to meet the customer's specifications?

What if the process has a Cp of 2.0, meaning that the process average will have to shift by 3 sigma units from the target value before it is likely that any non-conforming items will be produced. Should you relax the monitoring process because points which are only slightly out of control will have less of an impact?

The answer to both these questions is no. In the next section, we shall attempt to show why it is just as important to go on monitoring a process in the same way, no matter how good its capability index might be.