Rational subgrouping and reaction plans

Learning objectives

• Explain the purpose of rational subgrouping and reaction plans in a garment-factory improvement project.
• Apply the described method to a representative shop-floor situation.
• Recognize the common mistakes and how to avoid them.

Concept

Rational subgroups capture short-term common conditions so charts can reveal meaningful changes between periods.

Samples within a subgroup should be produced under similar conditions; separate subgroups across time/lot/shift to expose change.

Reaction plans distinguish containment, correction, investigation, verification, and escalation.

Garment-factory example

Take consecutive pieces from one machine at fixed intervals rather than mixing a whole shift into one subgroup.

Method

1. Document subgroup logic.
2. Establish sampling.
3. Train signal recognition.
4. Stop and contain when required.
5. Record cause and action.
6. Verify restart.

Common mistakes

• Subgrouping to hide variation across machines.
• Using many pattern rules without operational context.