Page 179 - Design for Six Sigma a Roadmap for Product Development

152 Chapter Five

5.6 Select the Best Concept
(DFSS Algorithm Step 5)
In this step, the DFSS team produces the convergence on the best con-
cept in iterative steps that are designed to be performed with DFSS
discipline and rigor. The following sequence may be used to facilitate
the convergence to the best concept by the DFSS team:

■ List criteria on the rows of the Pugh matrix [functional requirements
(FRs) array; constraints, regulatory and legal requirements from
phase 2 QFD mapping]. These criteria should be measurable and
defined with common understanding by all members of the team.
■ List concepts on the columns of the Pugh matrix as obtained from
the synthesis matrix.
■ Choose a datum design with which all other concepts are to be com-
pared from the alternative entities. The datum could be an existing
baseline, as is the case of incremental design. In creative design sit-
uations, the datum could be any concept that the team may generate
from the synthesis matrix. Evaluate concepts against the defined
criteria. Use a numbering system rather than the traditional evalu-
ation of plus ( ) and minus ( ). The datum will be the neutral ele-
ment(s) of the numbering system chosen. For comparing each
solution entity against the datum, rate either as plus ( ), meaning
better than the datum; or minus ( ), meaning worse than the
datum; or same (s), meaning same as the datum (see Fig. 5.11).
■ Perform trade-off studies to generate alternatives using design
axioms and TRIZ. Look at the negatives. What is needed in the design
to reverse the negative (relative to the datum)? Will the improvement
reverse one or more of the existing positives due to design coupling? If
possible, introduce the modified solution entity into the matrix and
retain the original solution entity in the matrix for reference pur-
poses. Eliminate truly weak concepts from the matrix. This will
reduce the matrix size. See if strong concepts begin to emerge from the
matrix. If it appears that there is an overall uniformity of strength,
this will indicate one of two conditions or a mixture of both. The cri-
teria are ambiguous and, hence, subject to mixed interpretation by the
DFSS team. Uniformity of one or more of the concepts suggests that
they may be subsets of the others (i.e., they are not distinct). In this
case, the matrix cannot distinguish where none exists.
■ Having scored the concepts relative to the datum, sum the ranks across
all criteria to get plus ( ), minus ( ), and (s) values. These scores must
not be treated as absolute as they are for guidance only and as such
must not be summed algebraically. Certain concepts will exhibit rela-
tive strengths, while others will demonstrate relative weaknesses.