Assumptions and method
Requires a stable process, suitable normal-distribution assumptions and a defensible within-process sigma estimate. Specification limits are not control limits.
Test a scenario
Use the result in a decision
Agree acceptance criteria and distinguish variation from a meaningful signal.
Requires a stable process, suitable normal-distribution assumptions and a defensible within-process sigma estimate. Specification limits are not control limits.
Set a baseline and change one assumption to compare outcomes.
A worked example
Follow the fixed teaching example
- Cp = (11 − 9)/(6 × 0.25) = 1.3333.
- Cpk = min(11 − 10.2, 10.2 − 9)/(3 × 0.25) = 1.0667. This requires a stable process and a meaningful spread estimate.
These illustrative inputs describe a project scenario, not a published benchmark. All monetary inputs use the same currency and price basis.
- Lower specification limit
- 9
- Upper specification limit
- 11
- Process mean
- 10.2
- Within-process sigma
- 0.25
- Cp
- 1.3333
- Cpk
- 1.0667
Interpret the output only within the assumptions above. Changing the inputs changes the result; it does not validate the inputs.
Source & credit
Standard process-capability indices. This implementation uses the published mathematical definitions without copying diagrams.
This is independently written code and explanation of the underlying method. The linked publication, its diagrams and its trademarks remain its owner’s material; no permission to reuse them is implied.