Quality Management
What the exam tests
Quality questions focus on in-process controls: SPC, specification versus control limits, capability indices, nonconformance handling, CAPA, sampling, holds, and quality gates.
Core rules
- Control limits come from process variation, usually mean +/- 3 sigma. Specification limits come from design or customer requirements. Never plot specification limits as control limits.
- Cp measures spread against specification width: (USL - LSL) / (6 sigma). Cpk also accounts for centering: minimum of (USL - mean)/(3 sigma) and (mean - LSL)/(3 sigma).
- A nonconformance record documents the deviation; a hold or quarantine prevents unintended use; CAPA addresses recurrence.
- Sampling plans trade inspection cost against escape risk. Reduced inspection should follow evidence, not convenience.
Worked example
Specification is 6 to 10 mm. The process mean is 8 mm and sigma is 0.5 mm. Cp = (10 - 6) / (6 x 0.5) = 4 / 3 = 1.33. Cpk = min((10 - 8)/(3 x 0.5), (8 - 6)/(3 x 0.5)) = min(1.33, 1.33) = 1.33. The process is centered and capable by this measure. If the mean shifted to 8.5 mm, Cp would remain 1.33 but Cpk would fall to 1.0, showing the centering loss.
Traps
- Concluding a process is capable from Cp alone when it is off-center.
- Treating an out-of-specification result as the only trigger for action; trends and runs matter.
- Closing a CAPA without root cause or effectiveness check.
- Releasing held material without documented disposition.