01 · Purpose
Why and when to use it
Classify variables before choosing charts, calculations or measurement-system studies.
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.
- Ask what each recorded value means.
- Distinguish categories from numerical quantities.
- For categories, check whether there is a meaningful order.
- For quantities, distinguish counts from continuous measurements.
- Record units, allowed values and suitable summaries in a data dictionary.
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.
| Variable | Type | Suitable first summary |
|---|---|---|
| Shift A/B | Nominal category | Counts or percentages by shift |
| Service rating 1–5 | Ordinal category | Category distribution; ordering retained |
| Errors per order | Discrete count | Count distribution and defects per unit |
| Packing time in minutes | Continuous measurement | Dot plot, median and spread |
04 · Interpretation
What the result tells you
The mean of shift codes 1 and 2 is meaningless. Whether recorded time is rounded to whole minutes does not change that duration is a continuous characteristic.
Completion check: A data dictionary that identifies each variable’s meaning and type.
05 · Judgement
Mistakes and limitations
Avoid treating ordinal ratings as equal-interval measurements without justification. Check whether zero is meaningful before interpreting ratios.
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.