3.1 Design Input & Requirements

3.1 Design Input & Requirements

Translating customer desires into robust, manufacturable engineering requirements is one of the most critical phases in product development. A failure at the design input stage cascades downstream, leading to costly design revisions, late-stage quality defects, and customer dissatisfaction. Quality engineers play a central role in structured requirement translation using tools like Quality Function Deployment (QFD) and the Kano Model.

---

1. Capturing the Voice of Customer (VOC) & Kano Model

Before technical specifications can be written, the Voice of the Customer (VOC) must be gathered through surveys, focus groups, customer interviews, warranty data, and field service reports. Customer feedback is often qualitative, vague, and subjective (e.g., "the car door should feel solid" or "the battery should last a long time").

To categorize customer expectations, Dr. Noriaki Kano developed the Kano Model, which classifies customer requirements into three distinct categories:

1. Basic / Must-Be Needs (Dissatisfiers): Requirements that customers take for granted. If present, customers are neutral; if absent or defective, customers are extremely dissatisfied (e.g., a car's brakes functioning properly).
2. Performance / One-Dimensional Needs (Satisfiers): Requirements where customer satisfaction is directly proportional to the level of performance. More is better (e.g., fuel economy, processing speed).
3. Delighters / Excitement Needs (Attractive): Unexpected features that surprise the customer. If absent, the customer does not complain because they did not expect it; if present, customer satisfaction rises dramatically (e.g., automated self-parking in early luxury vehicles).

                    Satisfied (Customer)
                            ^
                            |    / Delighters
                            |   / (Excitement)
                            |  / 
                            | /   / Performance
  Absent -------------------+----+-------------------> Fully Implemented
                            | /  /
                            |/  /  Basic Needs
                            /  /  (Must-Be)
                            | /
                            v
                  Dissatisfied

---

2. Quality Function Deployment (QFD) Framework

Developed by Yoji Akao and Shigeru Mizuno in Japan in the late 1960s, Quality Function Deployment (QFD) is a systematic planning methodology that translates customer requirements into technical requirements at each stage of product development. QFD cascades through four distinct phases:

---

3. Anatomy of the House of Quality (HOQ)

The primary tool of Phase 1 QFD is the House of Quality (HOQ), a matrix structure that resembles a building with a triangular roof. The HOQ consists of six major functional rooms:

                 +-----------------------+
                 |     ROOM 5: ROOF      |
                 | Correlation Matrix    |
                 +-----------------------+
                 |  ROOM 3: ENGINEERING  |
                 |     CHARACTERISTICS   |
+----------------+-----------------------+----------------+
| ROOM 1:        | ROOM 4: RELATIONSHIP  | ROOM 2:        |
| CUSTOMER       |        MATRIX         | CUSTOMER       |
| REQUIREMENTS   |                       | IMPORTANCE &   |
| (WHATS)        | (9-3-1 Weightings)    | BENCHMARKING   |
+----------------+-----------------------+----------------+
                 | ROOM 6: TECHNICAL     |
                 | PRIORITIES & TARGETS  |
                 +-----------------------+

• Room 1: Customer Requirements (Whats): Listed on the left rows. These represent customer expectations in their own words.
• Room 2: Customer Importance Ratings & Competitive Benchmarking: Column adjacent to Room 1. Customers rate the relative importance of each requirement on a 1-to-5 scale ($1 = \text{low}, 5 = \text{high}$).
• Room 3: Engineering Characteristics (Hows): Top horizontal columns. Technical design parameters that are measurable and controllable by engineering.
• Room 4: Relationship Matrix: The central grid connecting Whats (rows) to Hows (columns). Standard symbols represent relationship strength:
  • Strong Relationship ($\bullet$ or 9): High impact between requirement and specification.
  • Moderate Relationship ($\circ$ or 3): Medium impact.
  • Weak Relationship ($\Delta$ or 1): Low impact.
  • No Relationship (Blank or 0): No physical connection.
• Room 5: Correlation Matrix (The Roof): A triangular matrix mapping physical interactions between Engineering Characteristics. It identifies technical trade-offs ($--$, $-$) or positive synergies ($++$, $+$). For instance, increasing wall thickness improves structural rigidity ($+$) but increases overall vehicle weight ($-$ trade-off).
• Room 6: Technical Priorities, Targets & Benchmarking: Located at the bottom. Contains calculated Absolute Importance, Relative Importance (%), engineering units, and quantitative target values.

---

4. Step-by-Step Mathematical Construction of HOQ

Quality engineers must perform rigorous numerical calculations to prioritize engineering specifications based on customer weightings and relationship values.

Mathematical Formulation

For an HOQ matrix with $m$ Customer Requirements and $n$ Engineering Characteristics:

Let $d_i$ be the Customer Importance rating for requirement $i$ ($i = 1, 2, \dots, m$).
Let $w_{ij}$ be the relationship weight between requirement $i$ and engineering characteristic $j$ ($w_{ij} \in {0, 1, 3, 9}$).

The Absolute Importance ($AI_j$) for engineering characteristic $j$ is:

$AI_j = \sum_{i=1}^{m} (d_i \times w_{ij})$

The Relative Importance ($RI_j$) expressed as a percentage of total importance across all $n$ characteristics is:

$RI_j = \left( \frac{AI_j}{\sum_{k=1}^{n} AI_k} \right) \times 100\%$

---

Worked Numerical Example

Consider an engineering team designing an industrial handheld scanner. The customer requirements and relationship matrix are established below:

Step 1: Calculate Absolute Importance ($AI_j$)

• For Wall Thickness ($j=1$): $AI_1 = (5 \times 9) + (4 \times 9) + (5 \times 0) = 45 + 36 + 0 = 81$
• For Impact Resistance ($j=2$): $AI_2 = (5 \times 9) + (4 \times 1) + (5 \times 0) = 45 + 4 + 0 = 49$
• For Battery Capacity ($j=3$): $AI_3 = (5 \times 0) + (4 \times 3) + (5 \times 9) = 0 + 12 + 45 = 57$

Step 2: Sum Total Absolute Importance

$\sum_{k=1}^{3} AI_k = 81 + 49 + 57 = 187$

Step 3: Calculate Relative Importance ($RI_j$)

• Wall Thickness: $RI_1 = \frac{81}{187} \times 100% = 43.31%$
• Impact Resistance: $RI_2 = \frac{49}{187} \times 100% = 26.20%$
• Battery Capacity: $RI_3 = \frac{57}{187} \times 100% = 30.48%$

Engineering Decision: Wall Thickness ($43.31%$) emerges as the top technical priority, followed by Battery Capacity ($30.48%$). The design team must focus engineering resources on optimizing casing polymer selection and wall geometry while maintaining battery size constraints.

---

5. CQE Exam Tips & Key Pitfalls

[!IMPORTANT] On the ASQ CQE exam, remember that QFD is a team-based translation tool, not a voting system. Make sure you use standard weighting scales ($9-3-1-0$) when computing HOQ scores. Watch out for questions asking about the primary function of the HOQ Roof—its sole purpose is identifying physical trade-offs and correlations between technical specifications.

Design for X (DFX), Design for Six Sigma (DFSS), and Requirements Traceability

BoK Domain III.B.2 requires applying Design for X (DFX), Design for Six Sigma (DFSS), and requirements traceability.

Design for X (DFX)

DFX is a family of design disciplines that optimize a product/process for a named “X” objective while still meeting customer needs:

• DFM / DFA — Design for Manufacturability / Assembly (part count reduction, standard fasteners, poke-yoke assembly)
• DFS / DFR — Design for Serviceability / Reliability (access panels, derating, redundancy)
• DFE / DFT — Design for Environment / Testability (recyclability, built-in test points)

Design for Six Sigma (DFSS)

DFSS builds quality into new designs before production release, commonly using roadmaps such as DMADV (Define–Measure–Analyze–Design–Verify) or IDOV (Identify–Design–Optimize–Validate). Unlike DMAIC (improving an existing process), DFSS creates a capable design baseline.

Requirements Traceability

A traceability matrix links each customer/regulatory need → design input → design output → verification/validation evidence → production control. Gaps in the matrix are exam red flags: a CTQ with no verification method, or a process control with no linked design requirement.