01 · Purpose
Why and when to use it
Use this distinction before predicting conformity or reporting capability indices.
Who takes part?
The measurement owner works with people who understand the process and the recorded data. Statistical or measurement-system decisions should involve someone able to check the assumptions and study design.
02 · Application
How to apply it
Start with an agreed measurement purpose, explicit units and trustworthy observations. Identify missing cases and assumptions before calculating.
- Validate measurements and specification meanings.
- Inspect time-ordered data with an appropriate stability method.
- Investigate changes and define a defensible process population.
- Check distribution and sigma-estimation assumptions.
- Assess conformity and capability with uncertainty, distinguishing observed performance from future prediction.
03 · Example
A worked measurement example
Original fictional teaching example. Small datasets illustrate the method; they are not proposed as sufficient study sizes. Supplied parameters are assumptions, not estimates from a real process.
| Condition | Meaning | Next step |
|---|---|---|
| Stable, output too wide | Consistent process fails requirements | Improve the system and re-establish baseline |
| Unstable, few failures observed | Sample looks acceptable but behaviour changes | Investigate changes before relying on capability prediction |
| Stable and suitable spread/location | Predictive assessment may be defensible | Quantify uncertainty and maintain monitoring |
04 · Interpretation
What the result tells you
A control chart and a requirement comparison answer different questions. Never replace specification limits with calculated control limits to make the process appear acceptable.
Completion check: A requirement comparison supported by explicit stability and model checks.
05 · Judgement
Mistakes and limitations
Normal-model Cp/Cpk requires an appropriate within-process sigma estimate and distribution treatment. Finite data cannot prove that future conditions will remain unchanged.
07 · Reading
Sources and further reading
The tool list follows the accepted LeanTrainingLab overview and its reference to Six Sigma+Lean Toolset, second edition (Lunau, editor; Springer, 2013). The book chapters were not available for direct review during this build. Explanations, examples and worksheets are original teaching material. Further reading supplies professional background; local conventions are stated explicitly.