Process Capability Index (Cp and Cpk)
Reference model
Cp measures potential capability, while Cpk measures actual capability considering process mean shift.
About this model
Cp and Cpk are statistical measures of process capability, indicating how much a process varies relative to its specification limits.
Originator not stated.
How to read it
- Cp compares the width of the specification (USL minus LSL) with the spread of the process (6 standard deviations); a Cp above 1 means the spread fits inside the limits.
- Cpk is the smaller of the distances from the mean to each limit, divided by 3 standard deviations, so it also penalises a process that is off-centre.
- Cp equal to Cpk means the process is centred; a Cpk well below Cp means the mean has drifted toward one limit.
What this does
Measures how well a process can produce output within specified limits, indicating the capability of the process to meet customer specifications.
When to use it
Use it when assessing the performance of a process in relation to its specification limits, especially in manufacturing or quality control.
Not when: Do not use it when the process is not stable or when data is insufficient to calculate the indices accurately.
What happens next
Consider using the Control Chart tool to monitor process stability.
Golden thread
Builds on ←
- Process data collection
- Specification limits definition
Feeds into →
- Process improvement plan
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Drafted with AI and reviewed by Eccellere on 04/11/2026. Guidance, not advice: apply your own judgement.