A subgroup definition determines which variation is compared within groups and which variation appears between them.
Grouping is an engineering and statistical choice
Rational subgroups are intentionally constituted groups for process monitoring. Their definition should make the variation within and between groups meaningful to the intended question. A group of five rows is not automatically a suitable subgroup merely because the software requests a subgroup size.
A hypothetical group that mixes conditions
Suppose a five-reading group contains observations from five different approved machine configurations. Its within-group spread includes configuration differences. If the question concerns short-term variation within one configuration, that group answers a different question.
Alternatively, samples close in time from one condition may support a chosen within-condition comparison, while successive groups preserve longer-period changes. Whether that design is appropriate depends on actual operation and collection constraints. The illustration gives no prescribed HM group size.
Write the group rule before collection
Identify what one group represents: condition, time window, position or other experimental unit. Record what is held consistent and what intentionally differs between groups. Ensure the operator can label membership without guessing after collection.
Keep individual readings with subgroup IDs; group means alone conceal internal spread. If group sizes differ or a group crosses a transition, mark the deviation and agree its analysis treatment. A supplier reviewing a chart needs the grouping logic to interpret the variation estimate and any signal, rather than only a completed plot.
Trial discussion worksheet
This blank worksheet is for your own project. It contains no H M machine trial result.
| Review question | Reference or observation to retain |
|---|---|
| What does one group represent? | Condition and collection window |
| Which variation is within the group? | Explicit grouping rationale |
| Were group rules violated? | Identified membership deviations |
Customer Questions
Are consecutive rows always a rational subgroup?
No. Their collection conditions determine the meaning.
May group IDs be guessed after testing?
Record membership from the actual collection process.
Should individual values accompany group means?
Yes. Preserve the within-group evidence.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
Discuss Your Machine Requirement
Share your product, container, required output and the evidence needed for your technical review.
Request a Technical Review