Quality Assurance and Quality Control
The distinction
Quality control is product-focused and detection-based: inspection, testing, measurement, control charts, and disposition of nonconforming material. Quality assurance is process- and system-focused and prevention-based: quality planning, documented procedures, supplier qualification, training, document control, design review, and audits. Question stems usually describe an activity and ask you to classify it - if the activity finds a defect, it is QC; if it prevents defects or verifies the system, it is QA.
Cost of quality
Four categories: prevention (training, quality planning, supplier surveys, SPC implementation), appraisal (inspection, testing, calibration, audits), internal failure (scrap, rework, re-inspection, equipment downtime), and external failure (warranty, returns, recalls, liability, lost business). Common trap: rework is internal failure, not appraisal; calibration is appraisal, not prevention.
Corrective versus preventive action
Corrective action removes the cause of an existing nonconformity so it does not recur. Preventive action addresses a potential nonconformity that has not occurred. Root-cause tools include 5-Why, fishbone diagrams, Pareto analysis, and FMEA. Always verify effectiveness after implementation; closing an action without verification is a classic audit finding.
Audits
First-party audits are internal. Second-party audits are performed by a customer on a supplier, or by an organization on its suppliers. Third-party audits are performed by an independent body, such as an ISO 9001 registrar. Audits sample evidence - they do not inspect 100% of product.
Worked example
A plant installs an automated gauge at the end of the line to catch undersized parts. That is QC, and the cost is appraisal. The plant then adds a fixture in the press that makes undersizing physically impossible. That is QA, and the cost is prevention.