Quality function deployment

The House of Quality room by room, planning-matrix weights and improvement ratios, technical importance with 9-3-1 relationships, and the four phases of QFD.

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Why it matters

Customers describe what they want in their own words — "grinds fine", "quiet", "easy to clean". Engineers work with measurable characteristics — motor power, noise in dB, number of detachable parts. Quality function deployment (QFD) is the structured bridge between the two, so that design effort and money go to the characteristics customers actually care about, and conflicts between requirements are seen before tooling is made.

Key ideas

QFD was developed in Japan (Yoji Akao, Mitsubishi's Kobe shipyard, late 1960s–1970s) and spread through the automotive industry. It "deploys" the voice of the customer (VOC) through the whole development process using linked matrices.

House of Quality (HoQ) — the first and best-known matrix. Its "rooms":

  1. Customer requirements (WHATs) — the VOC, gathered by interviews, surveys and complaints, grouped into a hierarchy, each with an importance rating (typically 1–5).
  2. Technical (engineering) characteristics (HOWs) — measurable product properties that affect one or more WHATs, each with a direction of improvement (maximise, minimise, hit a target).
  3. Relationship matrix — how strongly each HOW influences each WHAT, usually strong = 9, medium = 3, weak = 1, blank = 0. A row with no strong relationship means a customer need is not being addressed; a column with no relationships suggests an unnecessary characteristic.
  4. Correlation matrix (roof) — how HOWs affect one another: positive (improving one helps the other) or negative (a trade-off, e.g. higher motor power raises noise). Negative correlations flag design conflicts to resolve — often with TRIZ-style thinking.
  5. Planning matrix (right wall) — customer competitive assessment: how customers rate our product and competitors on each WHAT, a target rating, the improvement ratio (target ÷ current), a sales point (about 1.0–1.5 for features that help sell the product), giving a raw weight and a normalised weight for each WHAT.
  6. Technical matrix (basement) — technical importance of each HOW (sum of weight × relationship down the column), relative importance in %, technical benchmarks of competitors, difficulty, and target values for each HOW.

Four-phase deployment. The HOWs of one matrix become the WHATs of the next:

  • Phase 1 product planning (HoQ): customer requirements → engineering characteristics.
  • Phase 2 part (design) deployment: engineering characteristics → part characteristics.
  • Phase 3 process planning: part characteristics → key process operations and parameters.
  • Phase 4 production planning: process parameters → production requirements (control plans, inspection, training).

Benefits and limits. QFD focuses design on customers, documents decisions, improves cross-functional communication and reduces late changes. It is time-consuming, depends on good VOC data, and large matrices become unmanageable — so teams limit it to the most important requirements.

Formulas

  • Improvement ratio: IR = target rating / current rating
  • Raw weight of a customer requirement: RW = importance × IR × sales point
  • Normalised weight: NW = RW / Σ RW × 100%
  • Technical (absolute) importance of characteristic j: TI_j = Σ wᵢ × rᵢⱼ
  • Relative technical importance: RTI_j = TI_j / Σ TI × 100%

Symbols: importance — customer importance rating (1–5); ratings — customer satisfaction scores on the same scale; sales point — dimensionless multiplier (1.0, 1.2 or 1.5); wᵢ — weight of WHAT i (either the importance rating or the normalised weight); rᵢⱼ — relationship strength (9, 3, 1 or 0).

Worked examples

Example 1 (standard) — technical importance. A mixer-grinder study uses three customer requirements with importance: grinds fine (5), quiet (3), easy to clean (4). Three technical characteristics: H1 motor power, H2 noise level, H3 number of detachable parts. Relationships:

  • Grinds fine: H1 = 9, H2 = 0, H3 = 1
  • Quiet: H1 = 3, H2 = 9, H3 = 0
  • Easy to clean: H1 = 0, H2 = 0, H3 = 9
  1. TI(H1) = 5×9 + 3×3 + 4×0 = 45 + 9 = 54.
  2. TI(H2) = 5×0 + 3×9 + 4×0 = 27.
  3. TI(H3) = 5×1 + 3×0 + 4×9 = 5 + 36 = 41.
  4. Total = 122; relative importance: H1 44.3%, H2 22.1%, H3 33.6%.

Motor power gets most design attention. The roof would show a negative correlation between H1 and H2 (more power, more noise) — a trade-off to resolve.

Example 2 (GATE level) — planning matrix then technical importance. Customers currently rate our product 3, 2 and 4 on the three requirements; targets are 4, 4 and 4; sales points are 1.5, 1.2 and 1.0.

  1. Improvement ratios: 4/3 = 1.333; 4/2 = 2.0; 4/4 = 1.0.
  2. Raw weights: 5 × 1.333 × 1.5 = 10.0; 3 × 2.0 × 1.2 = 7.2; 4 × 1.0 × 1.0 = 4.0. Sum = 21.2.
  3. Normalised weights: 47.2%, 34.0%, 18.9%.
  4. TI(H1) = 47.17×9 + 33.96×3 = 424.5 + 101.9 = 526.4; TI(H2) = 33.96×9 = 305.7; TI(H3) = 47.17×1 + 18.87×9 = 47.2 + 169.8 = 217.0.
  5. Relative: H1 50.2%, H2 29.1%, H3 20.7%.

Taking competition into account moves noise ahead of detachable parts: customers rate us poorly on quietness, so it now matters more than ease of cleaning, where we already meet the target.

Common mistakes

  • Multiplying importance by current satisfaction to get priority. A requirement already well satisfied needs less attention; the improvement ratio (target ÷ current) is what raises priority.
  • Writing solutions ("use a 750 W motor") as customer requirements, or vague HOWs that cannot be measured.
  • Summing technical importance across a row instead of down each column.
  • Ignoring negative correlations in the roof.
  • Using a 1-2-3 scale in one place and 1-3-9 in another within the same matrix.

For GATE PI

Expect MCQs on the rooms of the House of Quality, the meaning of the roof and the relationship symbols, the four phases of QFD, and the idea of the voice of the customer. Numericals ask for technical importance of a characteristic, an improvement ratio or a normalised weight. Practise column sums with 9-3-1 weights.

Quick check

  1. What does the roof of the House of Quality show?
  2. Importance 4, current rating 2, target 5, sales point 1.2: find the raw weight.
  3. Two WHATs with weights 5 and 2 relate to a HOW with strengths 3 and 9. Find its technical importance.
  4. In phase 3 of QFD, what are the HOWs?

Answers: 1. correlations (synergies and trade-offs) between technical characteristics; 2. 4 × 2.5 × 1.2 = 12; 3. 5×3 + 2×9 = 33; 4. key process operations and parameters.

Interview questions

All Engineering Economics and Product Design interview questions

Try answering each one aloud before you open it.

  1. 1.What is Quality Function Deployment (QFD)?Concept

    Quality Function Deployment (QFD) is a structured approach used in product design and development to ensure that the final product meets customer needs and expectations. It translates customer requirements into specific technical features and specifications. QFD uses a series of matrices to systematically link customer desires to engineering characteristics, ensuring that the voice of the customer is heard throughout the product development process.

  2. 2.Explain the main components of the House of Quality in QFD.Concept

    The House of Quality is a key component of QFD and consists of several parts: customer requirements (what the customer wants), technical descriptors (how the company will meet those needs), a relationship matrix (linking customer requirements to technical descriptors), a correlation matrix (showing how technical descriptors relate to each other), and competitive assessment (comparing the product with competitors). This structure helps in visualizing the relationship between customer desires and the company's ability to meet those desires.

  3. 3.How does QFD benefit the product development process?Application

    QFD benefits the product development process by ensuring that customer needs are prioritized and systematically addressed throughout the design and production stages. It helps in reducing the time to market by identifying potential issues early, improving communication among cross-functional teams, and enhancing product quality by focusing on customer satisfaction. By aligning technical specifications with customer requirements, QFD minimizes the risk of product failure in the market.

  4. 4.Why is customer feedback important in QFD?Application

    Customer feedback is crucial in QFD because it provides the foundation for identifying customer needs and expectations. This feedback is used to create the customer requirements section of the House of Quality, ensuring that the product development process is aligned with what customers actually want. By incorporating customer feedback, companies can prioritize features that add the most value to the customer, leading to higher satisfaction and competitive advantage.

  5. 5.What happens if a company ignores the correlation matrix in the House of Quality?Application

    Ignoring the correlation matrix in the House of Quality can lead to conflicts between different technical requirements, as it shows how these requirements interact with each other. Without understanding these interactions, a company might develop a product with conflicting features, leading to increased costs, delays, or a product that does not meet customer expectations. The correlation matrix helps in identifying and resolving potential conflicts early in the design process.

  6. 6.Describe a scenario where QFD might not be the best approach.Application

    QFD might not be the best approach in scenarios where the product development cycle is extremely short, and there is little time for detailed analysis and documentation. It may also be less effective for products with rapidly changing technologies or markets, where customer needs are not well-defined or are constantly evolving. In such cases, more agile and flexible development methodologies might be more appropriate.

  7. 7.How can QFD be integrated with other quality management tools?Application

    QFD can be integrated with other quality management tools such as Six Sigma, Total Quality Management (TQM), and Failure Mode and Effects Analysis (FMEA). For example, QFD can be used to identify critical customer requirements, which can then be further analyzed using FMEA to assess potential risks. Similarly, Six Sigma methodologies can be applied to optimize processes identified through QFD, ensuring that customer needs are met with minimal defects.

  8. 8.Calculate the raw weight of a customer requirement with importance 4, current customer rating 2, target rating 3 and a sales point of 1.5.Numerical

    Improvement ratio = target / current = 3 / 2 = 1.5. Raw weight = importance × improvement ratio × sales point = 4 × 1.5 × 1.5 = 9. Raw weights of all requirements are then normalised to percentages and multiplied by the 9-3-1 relationship strengths to rank the technical characteristics.

  9. 9.A customer requirement with importance 5 is linked to three technical characteristics with relationship strengths 1, 3 and 9. What does it contribute to each characteristic's technical importance?Numerical

    Its contributions are 5 × 1 = 5, 5 × 3 = 15 and 5 × 9 = 45 to the three characteristics respectively. Each characteristic's technical importance is the sum of such products down its own column over all customer requirements; the contributions are not added across the row, because that would mix different characteristics.

  10. 10.What role does competitive assessment play in QFD?Application

    Competitive assessment in QFD involves comparing the company's product with those of competitors to identify strengths and weaknesses. This assessment helps in understanding how well the product meets customer needs relative to competitors and can guide strategic decisions in product development. By analyzing competitive products, companies can identify opportunities for differentiation and improvement, ensuring that their product offers superior value to customers.

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