Quality Management
This domain tests whether you can run a quality management system, control a process statistically, judge a measurement system and sample a lot defensibly. The examiner wants clause knowledge plus the arithmetic of capability and sampling.
The management system
- ISO 9001:2015 is structured in clauses 4 to 10: context of the organisation, leadership, planning, support, operation, performance evaluation, improvement.
- Seven principles: customer focus, leadership, engagement of people, process approach, improvement, evidence-based decision making, relationship management.
- Risk-based thinking sits in clause 6.1 and replaces the old preventive action clause. Documentation is "documented information"; a quality manual is no longer mandatory, only the information the system needs.
- The process approach with PDCA is the audit lens: inputs, outputs, criteria, resources, measurement.
SPC and capability
- Variables charts: X̄-R for n = 2 to 10, X̄-s above that. Attribute charts: p, np, c, u.
- Control limits come from constants, not from the specification. For n = 5, A2 = 0.577, D4 = 2.114, D3 = 0.
- Run rules (Nelson/Western Electric) detect drift: 7 points one side of the centre line, 6 trending, 2 of 3 beyond 2σ.
- Cp = (USL − LSL)/6σ measures potential; Cpk measures actual centring. Ppk uses the overall standard deviation and answers a different question.
Sampling and measurement systems
- ISO 2859-1: AQL, general inspection level II, single sampling, with switching between normal, tightened and reduced. The OC curve shows producer risk at AQL and consumer risk at LTPD.
- Gage R&R: %GRR under 10% acceptable, 10–30% conditional, over 30% unacceptable; number of distinct categories should be 5 or more.
- Cost of quality follows the PAF model: prevention, appraisal, internal failure, external failure.
Worked example
A filling line is specified 500 ± 5 mL. The process runs at μ = 499 mL with σ = 1.2 mL. Cpk = min[(505 − 499)/3.6, (499 − 495)/3.6] = min(1.667, 1.111) = 1.111, below 1.33. Centring the mean at 500 mL gives Cp = 10/7.2 = 1.389, so a simple offset adjustment fixes the problem without touching the process spread.
Traps that cost marks
- Setting control limits at the specification limits.
- Quoting Cp when the process is off-centre and reporting it as capability.
- Choosing tightened inspection without the switching rules that justify it.
- Blaming operators for variation a gauge study shows is measurement error.