Quality Assurance and Control
What this domain tests
Quality Assurance and Control separates prevention from detection: the quality system, inspection and test planning, nonconformance handling, and statistical process control.
Core rules
- QA is process-focused (procedures, audits, supplier qualification). QC is product-focused (inspection, measurement, test).
- An inspection and test plan defines hold points (work stops until signed off), witness points (notify, may attend), review points and monitoring.
- Nonconformance handling: contain, disposition (use-as-is, rework, repair, reject), root-cause analysis, corrective action, then verify effectiveness.
- Control charts separate common-cause variation (change the process) from special-cause variation (investigate that point).
- Capability: Cp = (USL − LSL)/6σ and Cpk = min(USL − x̄, x̄ − LSL)/3σ.
Worked example
A bore has USL 10.20 mm, LSL 9.80 mm, process mean 10.05 mm and σ 0.03 mm. Cpk = min(10.20 − 10.05, 10.05 − 9.80)/(3 × 0.03) = min(0.15, 0.25)/0.09 = 1.67. The process is capable but off-centre: Cp = 0.40/0.18 = 2.22, so shifting the mean to 10.00 mm would raise Cpk towards 2.22 without any reduction in variation. That is the cheapest improvement available.
Traps
Calling inspection "QA"; calculating capability indices from a process that is not in statistical control (the indices are meaningless); closing an NCR with rework and no root cause; and treating ±3σ as if it were a tolerance.