Module 6 · Lesson 12 of 17
Capability confirmation after improvement
This lesson and the entire Lean Six Sigma Black Belt academy are free of charge — no fees, ever.
Learning objectives
- Explain the purpose of capability confirmation after improvement 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
Post-improvement capability confirms a stable process can meet specifications with enough margin under sustained conditions.
Cp/Pp assess spread; Cpk/Ppk include centering.
Use correct specifications, representative data, distribution review, confidence/uncertainty, and practical defect evidence.
Variables, units, and assumptions
Y in engineering units; USL/LSL; sigma_within (short-term) vs sigma_overall (long-term).
Process stable, roughly normal (or transformed), representative sample ≥ 30-50 subgroups.
Free-spreadsheet workflow
Free spreadsheet: Cp=(USL-LSL)/(6σ_within); Cpk=min((USL-μ),(μ-LSL))/(3σ_within); Pp/Ppk use σ_overall.
Interpretation and limitations
Cpk ≥ 1.33 typical minimum; ≥ 1.67 for critical CTQs.
Non-normal data need transform or non-parametric capability; small n gives unstable estimates.
Garment-factory example
Confirm chest measurement capability across sizes, lines, shifts, and material lots after pattern and method changes.
Method
- Confirm gauge and chart stability.
- Collect sustained data.
- Stratify.
- Calculate Cp/Cpk/Pp/Ppk in a free spreadsheet.
- Inspect tails.
- Compare with charter goal.
Common mistakes
- Computing capability on unstable pilot data.
- Pooling distinct size tolerances into one calculation.
Knowledge check
Pick one answer per question. Explanations appear after you submit.
1. High Cp with low Cpk indicates:
2. Capability calculation requires:
Author: NOSK.life · Last reviewed: 2026-07-21