Design for Six Sigma–Quality Function Deployment and the House of Quality


1. Quality Function Deployment

Quality control tries to reduce the defects in a manufactured product; quality assurance tries to put processes in place that will assure quality during the manufacturing process. What if quality can be designed into a product in the first place and not just assured and/or controlled during the manufacturing process?

Design for Six Sigma, then, is the third major topic in the Overview section of the Six Sigma Green Belt Certification body of knowledge, after Six Sigma and Lean Principles are introduced. One method for designing quality into a product is Quality Function Deployment, developed by Dr. Yoji Akao in Japan back in 1966. The idea behind Quality Function Development or QFD is to translate customer demands into specific technical design requirements that will, when deployed, achieve that quality demanded by the customer.

2.  The House of Quality

One tool that was developed to illustrate this method of QFD was the House of Quality, which first appeared in 1972. The reason why graphical tool is called the House of Quality is because it vaguely resembles a house. Here’s an example, and below it, I will explain the various features of the House of Quality or HOQ.

House of Quality Template

1. The left wall–VOC

This contains the customer requirements, which are sometimes called the “voice of the customer” or VOC. These are obtained through focus groups, surveys, or other methods to understand what potential customers would desire in a product or service. “Quality” in this sense simply means “what the customer wants.”    The relative importance of these customer requirements is given on the right-hand side of the left wall, usually with a scale from 1 to 5 with 1 meaning “not very important” to 5 meaning “very important”.   This becomes a factor in the “relative importance weight” of the design features which are shown as part of the foundation or basement of the house.

2. The right wall—Comparison with Competition

This contains the customer’s assessment of the competition. With relationship to the customer requirements listed in the left wall, how does the organization’s product or service stack up against the competition? Is it better, worse, or the same with regards to those requirements as compared to the competition?

3. The upper story—Technical Requirements

Here’s where the customer requirements get translated into details of the design. What are the technical requirements of the product or service? These are listed in the upper story above the main floor of the house (see paragraph 5 below).

4. The roof—Co-relationships between Technical Requirements

These technical requirements may reinforce each other, or they may conflict with each other. The price of components may be inversely related to their durability, for example. In the “roof” of the house, there are squares that relate to the intersection of various technical requirements in the upper story. Here is where you indicate whether the relationship is negative or positive, or whether it is strong, moderate, or weak.

5. Main floor—Relationship Matrix of the Technical Requirements

Here is where the Technical Requirements in the upper story of the house are ranked according to how well they actually achieve the customer requirements or elements of the VOC that are listed against the left wall.   They are usually given a weighting from 0 to 5, with 0 meaning “this design feature does not at all meet the customer’s requirements” to 5 meaning “this design feature totally meets the customer’s requirements.” It basically shows how strong the relationship is between the elements of the proposed design and the customer requirements in the VOC.   This relationship is another factor that is used in figuring out the “relative importance weight” of the design feature in the foundation or basement of the house.

6. Foundation—Target Values

The three elements of the foundation of the HOQ are distilled from the other elements. For each Technical Requirement, the following three values are derived, which are listed in the stories of the “foundation” or “basement” of the house.

1)  Relative Weight—the ranking of a) how well the Design Feature meets the needs of the customer requirement (on a scale from 0 to 5) multiplied times b) how important the Design Feature is from the customer’s standpoint.  The result is the “relative importance weight” of each design feature or “technical requirement”.  

2)  Benchmark Value—a measure of the specific value of the design element of the competitor’s version of the design, for example, “weighs < 5 lbs”.

3)  Target Value—a measure of the target value of the design element of the organization’s version of the design, for example, “must weigh < 4 lbs”. How much it should improve upon the benchmark value of the competition will depend on how much relative weight each element receives.

This gives a general idea of the House of Quality is laid out. It is a tool which shows visually how the customer’s requirements are mapped onto technical requirements which meet those requirements to a level exceeding that of the competition.

The next post in the Design for Six Sigma will discuss failure mode analysis.

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