01 · Purpose
Why and when to use it
Use this concept before comparing averages, reacting to individual results or assessing capability.
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.
- Define the measure and retain observation order and context.
- Plot results over time and inspect changes in level or spread.
- Choose an appropriate control chart when assumptions and data permit.
- Investigate signals with process knowledge rather than guessing from the pattern alone.
- Decide whether to address a specific event or improve the underlying system.
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.
| Pattern | Possible next investigation |
|---|---|
| Results fluctuate around a consistent level | Assess system performance and common-cause improvement opportunities |
| Sustained increase after a shift change | Check what changed and whether the measurement method also changed |
| One unusual delay | Trace the order and investigate its context without assuming a root cause |
04 · Interpretation
What the result tells you
A special-cause signal identifies evidence inconsistent with the modelled stable behaviour, not the cause itself. A stable process can still perform poorly against customer requirements.
Completion check: A reasoned view of process variation with evidence-based investigation priorities.
05 · Judgement
Mistakes and limitations
Do not adjust the process after every fluctuation. Independence, sampling and measurement errors affect pattern interpretation; unexamined mixtures can resemble instability.
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.