Quality Management and Continuous Improvement
This domain tests whether you can distinguish common from special-cause variation, read process capability correctly, and improve the process rather than inspect defects out.
Core rules
- PDCA drives improvement; ISO 9001:2015 uses the process approach and risk-based thinking across context, leadership, planning, support, operation, performance evaluation and improvement.
- Cost of quality: prevention and appraisal are deliberate spend; internal and external failure are the cost of getting it wrong, and external failure (field, recall, liability) usually dominates.
- Control charts separate common-cause from special-cause variation. Control limits come from the process (X̄ ± 3σ/√n on an X̄-R chart); specification limits come from the design. Never plot specification limits as control limits.
- Capability: Cp = (USL − LSL)/6σ measures spread only; Cpk = min(USL − μ, μ − LSL)/3σ also accounts for centring. Cp = Cpk only when perfectly centred.
- DMAIC structures Six Sigma projects; root cause via Ishikawa, 5-why and data, not opinion.
Worked example
A process runs μ = 50.00 mm, σ = 0.50 mm against limits 49.00–51.00 mm. Cp = 2.00/(6 × 0.50) = 0.667. Cpk = 1.00/(3 × 0.50) = 0.667 - centred but far too variable. To reach Cpk = 1.33 while centred, σ must fall to 1.00/(3 × 1.33) = 0.25 mm, halving the variation. That is a process change (tooling, feeds, setup control), not tighter inspection.
Traps
Confusing Cp with Cpk when the mean is off-centre; reacting to common-cause variation by tampering; treating control limits as specification limits; assuming more inspection improves capability; solving a symptom with 5-why that stops at human error.