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9609 · 9.2.1

Quality control and quality assurance — practice questions

Practice and worked examples for 9609 Quality control and quality assurance. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

Toy manufacturer relies on final inspection rejecting 5% of dolls before shipping. Defects still reach customers via sampling gaps. Recommend a quality approach.

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Current QC problem: 5% scrap cost, customer defects when sample misses faults - reactive.

QA shift: Supplier quality standards, in-line checks at moulding stage, staff training - prevent defects at source.

TQM elements: Team quality circles, customer complaint analysis, Kaizen (9.3.4).

Cost-benefit: Upfront training/ISO cost vs lower scrap, fewer returns, protected brand - usually net positive for consumer goods.

Worked example 2

Precision Parts Ltd. produces 100,000 components per month. Its current Quality Control system identifies a 4% defect rate, costing $15 per unit to rework. A further 1% of units have defects missed by QC and reach customers, costing $50 per unit in warranty and return costs. The company is considering a new Quality Assurance system costing $25,000 per month, which it believes will reduce the internal defect rate to 0.5% and the external defect rate to 0.1%. Calculate the net monthly financial benefit of implementing the QA system.

Show solution outline

Step 1: Calculate the current monthly Cost of Poor Quality (CoPQ) with QC only.

  • Internal Failure Cost (Rework): This is the cost of fixing defects found before they reach the customer. Units produced×Internal defect rate×Rework cost per unit\text{Units produced} \times \text{Internal defect rate} \times \text{Rework cost per unit} 100,000×4%×$15=4,000×$15=$60,000100,000 \times 4\% \times \text{\textdollar}15 = 4,000 \times \text{\textdollar}15 = \text{\textdollar}60,000

  • External Failure Cost (Warranty/Returns): This is the cost of defects found by the customer. Units produced×External defect rate×Return cost per unit\text{Units produced} \times \text{External defect rate} \times \text{Return cost per unit} 100,000×1%×$50=1,000×$50=$50,000100,000 \times 1\% \times \text{\textdollar}50 = 1,000 \times \text{\textdollar}50 = \text{\textdollar}50,000

  • Total Current CoPQ: $60,000+$50,000=$110,000 per month\text{\textdollar}60,000 + \text{\textdollar}50,000 = \text{\textdollar}110,000 \text{ per month}

Step 2: Calculate the new monthly Cost of Quality with the QA system.

This includes the investment in QA (prevention cost) plus the reduced failure costs.

  • Prevention Cost (QA System): $25,000 per month\text{\textdollar}25,000 \text{ per month}

  • New Internal Failure Cost: 100,000×0.5%×$15=500×$15=$7,500100,000 \times 0.5\% \times \text{\textdollar}15 = 500 \times \text{\textdollar}15 = \text{\textdollar}7,500

  • New External Failure Cost: 100,000×0.1%×$50=100×$50=$5,000100,000 \times 0.1\% \times \text{\textdollar}50 = 100 \times \text{\textdollar}50 = \text{\textdollar}5,000

  • Total New Cost of Quality: Prevention Cost+Internal Failure+External Failure\text{Prevention Cost} + \text{Internal Failure} + \text{External Failure} $25,000+$7,500+$5,000=$37,500 per month\text{\textdollar}25,000 + \text{\textdollar}7,500 + \text{\textdollar}5,000 = \text{\textdollar}37,500 \text{ per month}

Step 3: Calculate the net monthly financial benefit.

  • Net Benefit: This is the difference between the old total cost and the new total cost. Total Current CoPQ−Total New Cost of Quality\text{Total Current CoPQ} - \text{Total New Cost of Quality} $110,000−$37,500=$72,500\text{\textdollar}110,000 - \text{\textdollar}37,500 = \text{\textdollar}72,500

Final Answer: The net monthly financial benefit of implementing the new Quality Assurance system is $72,500.

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