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Quality Function Deployment (QFD)

Quality function deployment (QFD) is the structured methodology used to map CT characteristics into design specifications, i.e., KPOVs or “Ys.” QFD is also used to analyze the performance of KPOVs to targets and current system performance to identify gaps. Information technology

FIGURE 3.10

House of quality. CT = critical-to characteristic; KPOV = key process input variable; Y = outputs; X = inputs.

facilitates internal and external benchmarking. Mapping is useful in coordinating communication between the design team and other organizational functions. This ensures the linkage of the VOC and KPOVs to designs and processes. The QFD methodology is organized as the “house of quality” (HOQ) shown in Figure 3.10. The HOQ is divided into sections or rooms that summarize information relevant for understanding relationships between customer requirements (i.e., KPOVs and system elements of the product or service design).

The CT characteristics drive the internal specifications corresponding to KPOVs. These KPOVs are refinements of a CT characteristic (e.g., delivery time measured in days starting at time A and ending at time B). Recall that these are initially higher-level ideas. The analysis focuses general concepts into quantified metrics. When their performance is compared to targets product or process performance gaps are identified that require that new or modified systems be created.

These KPOVs are further defined as one or more specifications, or Ys that in aggregate satisfy the CT expectations of the customer. They are controlled by key process input variables (KPIVs), i.e., “Xs” that drive the levels of the Ys. This is the Six Sigma model relationship Y =/(X), where we understand how the level of Y changes as inputs (X) change. The goal of mapping the KPOV to specifications is to develop these quantitative relationships or models between the Ys and their associated inputs (or Xs). The HOQ enables a team to see interrelationships between several KPOVs and the variables driving them. In addition, it shows current gaps in the sub-systems producing features and functions (i.e., the Ys as well as the Xs). This relationship information is useful when making design tradeoffs between the sub-systems. As an example, one KPOV may be that an automobile has a fuel economy of 25 miles per gallon as well as a minimum weight for road handling and safety. The HOQ would show interrelationships between sub-systems and their Ys that would provide tradeoffs between mileage and weight.

Section A of Figure 3.10 prioritizes the CT characteristics represented as KPOVs. The relative importance ratings (i.e., the prioritization) are estimated using prioritization tools that will be discussed in Chapter 4. In QFD literature, these CT characteristics are called the whats. In section В of Figure 3.10, the design requirements (i.e., the Ys) are listed as specifications. These Ys are called the hows. Section C of Figure 3.10 shows correlations, if they exist, between each KPOV and the Ys (i.e., correlations between the whats as they relate to the hows). A rating system of 1 to 10 is used to indicate weak (1), medium (5), and strong (10) correlations between the whats and the hows. Competitive benchmarking, shown in section D, is also used to aid the analysis. Section E is used to evaluate relationships between one or more design elements (i.e., the Ys or the hows) because there may be design conflicts, again on a scale of 1 to 10. A rating of 10 implies a high correlation between design elements or Ys. The impact of this correlation could be positive or negative. This information aids tradeoff decisions. Section F lists the performance targets for the Ys. Several Ys may be required to satisfy a specific KPOV or CT characteristic. In section G, specifications are compared to the current design’s capability to identify performance gaps. Performance gaps require that one or more projects be deployed to improve system performance. Alternatively, entire new systems may need to be created.

FIGURE 3.11

How to use a SIPOC to translate the VOC. SIPOC = supplier, input boundary, process, output boundary, and customer; QFD = quality function deployment; VOC = voice of the customer.

Service systems can also be designed using the QFD methodology, although process mapping at various levels and of different types is usually more convenient. Mapping a process begins with identifying the project’s scope, which in turn helps determine where the process lies and its beginning and end points. Figure 3.11 shows a high-level map or SIPOC, which is an acronym for supplier, input boundary, process, output boundary, and customer. A SIPOC analysis captures the prioritized list of KPOVs or Ys. This information is used to associate them to process steps or operations and the Xs that control the Ys. The SIPOC chart becomes more detailed and quantified until a system model can be created to quantitatively describe the relationships of process inputs and outputs (i.e., Y =f{X). Depending on the project, a high-level description may be adequate or a more detailed process maps be needed. If the quantification is complete, a system model can be created, and simulations can also be developed by varying the input levels.

 
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