KEVOS® MAKEOS · Quality
Process Capability from Summary Data
What Cp and Cpk follow from this mean and within-process spread?
Calculate two-sided Cp and Cpk from supplied specifications, process mean and within-process standard deviation.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Lower specification | unit | Engineering specification in the same unit as all other dimensional inputs. |
| Upper specification | unit | Upper engineering specification; not a control limit. |
| Process mean | unit | Mean from a representative stable process dataset. |
| Within-process standard deviation | unit | Appropriate within-subgroup estimate; not tolerance divided by six. |
| Stable process and suitable distribution confirmed? | choice | Review control charts, sampling and the distribution before interpreting capability. |
Method and formulas
- Cp = (upper specification − lower specification) / (6 × within-process standard deviation).
- Cpk = minimum of [(upper − mean)/(3σ), (mean − lower)/(3σ)].
- Example: 9.9–10.1 limits, mean 10.02 and σ = 0.02 gives Cp = 1.666667 and Cpk = 1.333333.
Limitations
- No stability test, sigma estimation, confidence interval or defect-rate forecast is performed.
- If the spread is an overall sample standard deviation, the corresponding indices are Pp/Ppk, not Cp/Cpk.
- No universal capability acceptance threshold is assumed.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Cp Cpk
- process capability
- quality statistics
Who it is for
- Quality engineers
- Manufacturing engineers
What you get
- Cp
- Cpk
- Upper-side index
- Lower-side index
Interpreting the result
- Negative Cpk means the mean lies outside a specification limit. Compare with an agreed customer criterion only after validating the study.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 128–133. Topic reference; no source tables or recommended settings are reproduced.
- NIST/SEMATECH e-Handbook: What is Process Capability? — https://itl.nist.gov/div898/handbook/pmc/section1/pmc16.htm
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
