Process Capability Guide

Cp index

Cp index is a statistical measure of process capability. This means that it is a measure of how capable a process is of producing output within specification limits. This measurement only has meaning when the process being examined is in a state of statistical control. Cp disregards centering, and is insensitive to “shifts and drifts” (special cause) in the data. If the process mean is not centered within the specification limits, this value may therefore be misleading, the Cpk index should be used instead for analyzing process capability instead:

(where USL is Upper Spec Limit and LSL is Lower Spec Limit and sigma is the standard deviation of the process)

Cpk index

Cpk index is a type of process capability index. It is sensitive to whether the process is centered, but insensitive to special cause. As a formula:

(where USL is Upper Spec Limit and LSL is Lower Spec Limit and sigma is the standard deviation of the process)

In simplest terms, Cpk indicates how many times you can fit three standard deviations of the process between the mean of the process and the nearest specification limit. Assuming that the process is stable and predictable, if you can do this once, Cpk is 1, and your process probably needs attention. If you can do it 1.5 times, your process is excellent, and you are on the path to being able to discontinue final inspection. If you can do it 2 times, you have an outstanding process. If Cpk is negative, the process mean is outside the specification limits.

Tips

1. Cp does not change as the process center changes.
2. Cp = Cpk when the process is centered.
3. Cpk is always (
4. A Cpk value of 1.00 is a de facto standard. It indicates that the process is producing product that conforms to specifications and is on target.
5. A Cp value less than 1.00 indicates a process whose average is not centered and whose parts may not be conforming to the specification.
6. A Cpk of less than 1.00 indicates a process producing product not conforming to specifications.
7. A Cpk value of zero indicates that the average (X-bar-bar) is equal to one of the spec limits.
8. A negative Cpk indicates that the average is outside the specifications.

Pp and Ppk

As their description implies, Pp and Ppk look at what the performance could be. Some people refer to them as short-term capability indices because they do not look at process variation over time. Another use of the Process Performance Indicators today is to get a look at how the total variation from the process compares to the specification. The formulas for Pp and Ppk are quite similar to those for Cp and Cpk. The big difference between them is how we calculate the sample standard deviation, s.