INTRODUCTION TO SOFTWARE DESIGN FOR SIX SIGMA (DFSS) - Software Design for Six-Sigma: A Roadmap for Excellence - page 190

design, a fundamental set of principles that determine good design practice, can help
to facilitate a project team to accelerate the generation of good design concepts. Ax-
iomatic design holds that uncoupled designs are to be preferred over coupled designs.
Although uncoupled designs are not always possible, application of axiomatic design
principles in DFSS presents an approach to help the DFSS team focus on functional
requirements to achieve software design intents and maximize product reliability. As
a result of the application of axiomatic design followed by parameter design, a robust
design technique, the DFSS team achieved design robustness and reliability.
Design for X-ability (DFX) 22 is the value-added service of using best practices
in the design stage to improve X where X is one of the members of the growing
software DFX family (e.g., reliability, usability, and testability). DFX focuses on
a vital software element of concurrent engineering, maximizing the use of limited
recourses available to the DFSS teams. DFX tools collect and present facts about
both the software design entity and its production processes, analyze all relationships
between them, and measure the CTQ of performance as depicted by the concep-
tual architectures. The DFX family generates alternatives by combining strength
and avoiding vulnerabilities, provides a redesign recommended for improvement,
provides an if-then scenario, and does all that in many iterations.
A gap analysis identifies the difference between the optimized allocation and in-
tegration of the input and the current level of allocation. This helps provide the team
with insight into areas that could be improved. The gap analysis process involves
determining, documenting, and approving the variance between project requirements
and current capabilities. Gap analysis naturally flows from benchmarking and other
assessments. Once the general expectation of performance in the industry is under-
stood, it is possible to compare that expectation with the current level of performance.
This comparison becomes the gap analysis. Such analysis can be performed at the
strategic or operational level of an organization.
Robust Design 23 variation reduction is recognized universally as a key to reliability
and productivity improvement. There are many approaches to reducing the variability,
each one having its place in the product development cycle. By addressing variation
reduction at a particular stage in a product's life cycle, one can prevent failures
in the downstream stages. The Six Sigma approach has made tremendous gains in
cost reduction by finding problems that occur in operations and fixing the immediate
causes. The robustness strategy of the CTQs is to prevent problems through optimizing
software product designs and their production operations.
Regression is a powerful method for predicting and measuring CTQ responses.
Unfortunately, simple linear regression is abused easily by not having sufficient
understanding of when to—and when not to—use it. Regression is a technique that
investigates and models the relationship between a dependent variable (Y) and its
independent predictors (Xs). It can be used for hypothesis testing, modeling causal
relationships (Y f (x)), or a prediction model. However, it is important to make sure

that the underlying model assumptions are not violated. One of the key outputs in a

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22 See Chapter 14.
23 See Chapter 18.