Quality Concepts and Tools
Quality is defined both as conformance to requirements (Crosby) and fitness for use (Juran); scenario questions often hinge on which definition applies. The cost of quality model splits spending into prevention (training, supplier qualification, design review, quality planning), appraisal (inspection, test, audit), internal failure (scrap, rework, downtime caught before shipment) and external failure (returns, warranty, recalls, liability). Failure costs are the target; prevention is usually the cheapest dollar spent. Watch the classic traps: a complaint is external failure even if resolved free of charge, and rework found in-house is internal failure no matter how expensive.
The seven basic QC tools are check sheet, histogram, Pareto chart, cause-and-effect (fishbone) diagram, scatter diagram, flowchart/stratification, and control chart. Know which tool answers which question: Pareto prioritizes the vital few, fishbone generates candidate causes, scatter tests correlation (never causation), and a control chart separates common from special cause.
Improvement frameworks: PDCA/PDSA, Six Sigma DMAIC, Lean (waste, 5S, kaizen, value stream mapping), benchmarking, QFD (house of quality, translating customer needs into design characteristics), and FMEA (RPN = severity × occurrence × detection).
Worked example. A plant spends $40k on training, $60k on inspection, $25k on in-house rework, and $90k on warranty claims. Prevention = $40k; appraisal = $60k; internal failure = $25k; external failure = $90k; total = $215k, with failure costs at 53% of the total - the classic signal to shift spending upstream.