Quality Management and Continuous Improvement
This domain tests quality systems and improvement methods: prevention over detection, variation, root cause, control charts, capability, corrective action, and audit discipline. Expect interpretation of data and selection of the next quality action.
Core rules and concepts
- Quality assurance builds confidence through planned processes; quality control evaluates outputs against criteria.
- PDCA and DMAIC provide improvement cycles; define the problem, measure current state, analyze causes, improve, and control.
- Distinguish common-cause variation from special-cause signals; do not adjust a stable process for every point.
- Cp=(USL−LSL)/(6σ) measures potential capability; Cpk=min(USL−mean, mean−LSL)/(3σ) accounts for centering.
- Corrective action removes root cause; containment protects the customer in the interim.
- Audit findings need objective evidence, risk-based prioritization, and verified closure.
Worked example
Specification 9.8–10.2 mm, mean 10.05 mm, σ=0.05 mm. Cp=0.4/(6×0.05)=0.4/0.30=1.33. Cpk=min(10.2−10.05, 10.05−9.8)/(3×0.05)=min(0.15,0.25)/0.15=1.00. Reasoning: the process is potentially capable but off-center; centering at 10.0 would raise Cpk toward 1.33.
Traps that cost marks
- Reporting Cp as capability when the mean is off target.
- Using specification limits as control limits.
- Treating containment as permanent corrective action.
- Acting on every common-cause fluctuation.
- Estimating σ from too few subgroups.