Process Capability (Cp) and Performance (Cpk) Chart

Capability (Cp) and performance (Cpk) charts illustrate a process’s ability to meet specifications.

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

The Cp Chart

The SPC Cp measurement shows how well the Six Sigma range fits into the specification range. This measurement is determined by dividing the specification limit (voice of the customer) by the control limit (voice of the process).

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

The Cpk Chart

The SPC Cpk measurement shows the relationship of the Six Sigma spread 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 the Cpk measurement, compare the average of the data to the specification limits.

                 The Cpk Process Capability Charts

Evaluating the Relationship Between Cp and Cpk

Evaluating the Relationship CP VS CPK
You can use capability and performance charts to evaluate process performance and acceptability. Before performing this type of capability study:
  • Verify that process variability is stable.
  • Review individual data values on a histogram chart to verify that the distribution of values 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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