Page 126 - Design for Six Sigma a Roadmap for Product Development

Product Development Process and Design for Six Sigma 101

3.11 Design for a Six Sigma
(DFSS) Company
A company is Six Sigma–capable when each of its products and services
achieves 3.4 defects per million long-term capabilities, a Six Sigma level
assuming normality. Over the course of a life cycle, including develop-
ment, thousands of decisions are taken to design, produce, release, and
service the entity. A critical few are taken at the milestones, but the
trivial many are taken on a daily basis. A design decision may be con-
forming or nonconforming. Nonconformance in the development process,
as used here, has a broader meaning than the intuitive definition of error.
Nonconformity occurs in design development when a decision produces
less-than-ideal results. It could be a wrong action or a missed inaction. In
either case, it can be committed as one of the following scenarios: when
a necessary assurance decision is not executed, when a necessary mea-
sure is pursued inadequately, and when an important matter is not
addressed or is addressed inappropriately. These decisions and an
assessment of their goodness in light of the nonconformance definition
cited above must be recorded. This will allow the company to decide on
the right direction and to avoid future pitfalls. A DFSS company has
3.4 nonconfirming decisions per million. A vehicle, for example, has
thousands of parts, which translates into millions of decisions con-
cerning concept, performance, quality, appearance, cost, and other
variables of the design entity over the course of different development
phases. A company can chart its performance in a p-chart over the

development milestones. The centerline, p, can be established from
company history. Let the total number of decisions taken at milestone
k equal n k and the number of nonconfirming decisions equal to D k ; then
^
the milestone proportion nonconfirming is given as p (D k /n k ). The
Z-score sigma control limits are given by

(1
)

Z n k
For example, a 3 (6 ) limit can be obtained when Z equals 3 (6),
respectively. Charting starts by drawing the control limits and plotting
^
the current milestone
.
3.12 Features of a Sound DFSS Strategy

The DFSS team will have a first-time real-world useful feedback about
their design efforts in the prototype phase based on testing and perfor-
mance in the working environment. This usually happens almost after
the middle of the development cycle. As such, there is not much room to