Cp and Cpk are process capability indices that show a process’s ability to produce product within tolerance limits.
The difference
Cp compares the spread of the process with the width of the tolerance. It does not look at whether the process is centred; it measures only the potential.
Cpk also takes centring into account. If the process mean has shifted from the centre of the tolerance, Cpk drops.
How is it interpreted?
If Cp is high but Cpk is low: the spread of the process is adequate, but the centre has shifted. This is good news, because adjusting the centre is usually easier than narrowing the spread.
If both are low, the spread needs to be reduced, and that is a more demanding job.
Precondition
The capability calculation is not made until the process has been confirmed to be in statistical control with a control chart. In addition, if the reliability of the measurement system has not been verified with MSA, the calculated index may be mistaking measurement noise for process variation.
For the counterparts calculated with long-term data, see the Pp and Ppk indices.
Calculating capability indices, when to use which, and reporting them are addressed hands-on in the SPC training. A combined programme with the MSA training is recommended for measurement reliability.