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Process Capability (Cp)
and Performance (Cpk)

Learn how to determine whether your process is meeting its full potential—and see opportunities for improvement.
 

How do you use Cp and Cpk?

Although statistical process control (SPC) charts can reveal whether a process is stable, they do not indicate whether the process is capable of producing acceptable output—and whether the process is performing to potential capability.

Capability (Cp) and performance (Cpk) charts go beyond elemental quality control to illustrate a process’s ability to meet specifications. Using information from these statistics, you can better understand which processes need improvement, where you have opportunities for improved productivity, and how to prioritize improvement activities.
 

Let’s take a look at the difference between Cp and Cpk.

What is Cp?

The Cp ratio shows how well the process spread (expressed as six standard deviations) fits into the specification range. This measurement is determined by dividing the specification limit (voice of the customer) by the process spread (voice of the process).

To calculate Cp, subtract the lower specification limit from the upper specification limit, then divide by six standard deviations.

What is Cpk?

The Cpk ratio shows the relationship of the process spread to the specification limits while taking into account the centering of the process compared to the specification limits. Cpk represents the lowest value of the capability against the upper or lower specification, showing where, within the specification limits, the process is producing.

To calculate Cpk, compare the average of the data to both the upper and lower specification limit. An off-centered process will have a greater risk of fallout to the specification limit closest to the process mean. The reported Cpk will be the one that measures the highest risk.

                 The Cpk Process Capability Charts
Evaluating the Relationship CP VS CPK

Evaluating the Relationship Between Cp and Cpk

Before relying on the Cp and Cpk values:
  • Verify that process variability is stable, i.e. no out-of-control patterns on the control charts.
  • Review individual data values on a histogram chart to verify that the distribution is normal (or close to normal).
  • Verify that engineering tolerances are known.
  • Verify that the estimated standard deviation of the process is known.
Never attempt to interpret numerical summaries of capability without also looking at a histogram of the data plotted against specification limits. Capability studies should also include analysis of control charts and capability indices.

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See the Cp and Cpk Charts in Action

When you use SPC software from InfinityQS, determining capability becomes easier than ever. See how to turn Cp and Cpk chart data into actionable information.
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