10.4 Quality Function Deployment & the House of Quality

10.4 Quality Function Deployment & the House of Quality

Body of Knowledge entry V.B.3 (Customer needs) states: understand the key elements of quality function deployment (QFD) and how it identifies and prioritizes customer expectations and needs. Customer Relationship is only 7 scored questions, and QFD is one of its four entries — so this is worth roughly one to two items, and it is a topic candidates routinely skip.

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1. Purpose

Quality function deployment (QFD) is a structured method for translating the voice of the customer into measurable technical requirements, and for prioritizing those requirements according to what customers actually value.

It solves a specific and expensive problem. Customers express needs in their own language — "the door should feel solid," "the software should be easy to learn," "the coffee should stay hot." Engineers can only act on measurable characteristics — closing force in newtons, time-to-first-successful-task in minutes, temperature retention in degrees per fifteen minutes. Without a disciplined translation, one of two failures occurs: engineers optimize what is easy to measure rather than what customers value, or design decisions are made by whoever argues most persuasively in the meeting.

QFD was developed by Yoji Akao in Japan in the mid-1960s, with its first large-scale application at the Mitsubishi Heavy Industries Kobe shipyard in 1972. It reached Western industry through the automotive sector in the 1980s.

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2. The House of Quality

The House of Quality (HOQ) is QFD's first and best-known matrix, named for the roof-shaped correlation triangle on top. Its rooms:

                        +-------------+
                        |    ROOF     |  Correlations among the HOWs
                        | (trade-offs)|  (+ synergy / - conflict)
                +-------+-------------+-------+
                |        HOWs (technical       |
                |        requirements)         |
        +-------+------------------------------+--------+
        | WHATs |                              | Customer|
        |  +    |     RELATIONSHIP MATRIX      | compet. |
        | Imp.  |     (9 / 3 / 1)              | assess. |
        +-------+------------------------------+--------+
                |  Technical priorities,       |
                |  targets, benchmarks         |
                +------------------------------+

Calculating Technical Priority

Relationships are scored with the conventional weights:

For each technical requirement, multiply its relationship weight to each customer requirement by that requirement's importance rating, then sum down the column:

$\text{Absolute importance}_j = \sum_i (\text{Customer importance}_i \times \text{Relationship}_{ij})$

Worked example. A HOW ("insulation R-value") relates strongly (9) to a WHAT rated 5 in importance, moderately (3) to a WHAT rated 4, and weakly (1) to a WHAT rated 2:

$(5 \times 9) + (4 \times 3) + (2 \times 1) = 45 + 12 + 2 = \mathbf{59}$

Ranking all columns by this score tells engineering where design effort buys the most customer-perceived value. This is the "prioritizes" half of the BoK entry, and it is the output that matters.

The non-obvious diagnostic is a blank row or a blank column:

• A blank row means a customer requirement no technical characteristic addresses — the design will not deliver something customers asked for.
• A blank column means a technical requirement that serves no customer requirement — effort and cost with no customer value, and a candidate for elimination.

The Roof and Design Trade-offs

The roof records how the technical requirements interact. A positive correlation means improving one helps another. A negative correlation means they conflict: increasing wall insulation raises thermal performance and increases weight and cost; tightening a manufacturing tolerance improves fit and slows cycle time.

Surfacing negative correlations is one of QFD's most valuable outputs, because these are the trade-offs that otherwise get discovered late — after tooling, when resolving them is expensive. The roof forces them into an explicit, documented decision made early.

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3. The Four-Phase Model

The House of Quality is phase one. The classic four-phase deployment carries the customer's voice all the way to the production floor by a simple recursive rule: the HOWs of one matrix become the WHATs of the next.

The payoff is traceability: by phase four, a control on the shop floor can be traced back through process parameters and part characteristics to the specific customer requirement it protects. When a control plan is questioned, QFD answers which customer need it exists to serve.

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4. Benefits and Limitations

Benefits: it forces cross-functional agreement early; it makes design priorities explicit and defensible instead of political; it front-loads effort into design where change is cheap; it surfaces trade-offs and gaps that would otherwise appear late; and it creates a documented link from every control back to a customer need.

Limitations: a full four-phase deployment is time-consuming and matrices grow large quickly; the analysis is only as good as the underlying VOC data; and importance ratings are judgments that can be manipulated.

QFD and the Kano Model

The two are complements, and the exam may pair them. Kano (section 10.2) classifies requirements as must-be, one-dimensional, or attractive. QFD translates and prioritizes them. Kano feeds QFD: must-be requirements need no importance rating because their absence is disqualifying, whereas attractive requirements are systematically under-rated in direct surveys because customers do not ask for what they have not imagined. Feeding raw survey importance scores into a House of Quality without a Kano screen predictably under-weights the delighters and over-weights the expected basics.