As stated in the section on capability, the index Cp is only a measure of process potential. If the process continues to operate in a state of statistical control, and the process mean is on, or very close to the nominal value, then we can be confident that only a tiny fraction of the process' outputs may lie outside the specification limits. However, it's quite possible for a process with a much higher Cp value to produce non-conforming product, if it is not well-centred with respect to the nominal value. In order to check for this condition, we need to calculate another capability index Cpk.
Cpk takes into account both: