Page 33 - Biomedical Engineering and Design Handbook Volume 2, Applications

12 MEDICAL DEVICE DESIGN

recognized immediately as inappropriate. (This is particularly true if the team is diligent about
seeking ideas.) The notably bad ideas should be quickly but carefully (and sometimes even gently)
culled out and discarded.
The second issue revolves around selection criteria. The process will (or should) pick the idea that
provides to the team and the organization the best opportunity to develop a successful product. In
making that selection, the capabilities of the organization come into consideration. What kind of
products is the group experienced with? What kind of manufacturing facilities does it have at hand?
What are the core competencies of the corporation? The concern here is the good idea that doesn’t
fit. As a simplistic example, if the organization builds mechanical things, and the best concept for
the product under discussion is an electronic solution, there is an impasse. The kind of design avail-
able is second rate, and will fail in the marketplace. (Someone will market the electronic version!)
The options are (1) abandon the product plan, (2) acquire the needed skills and competency, or
(3) partner with one or more organizations that can provide the talent. Contracting with a design firm
for development and an outside manufacturing organization might accomplish the latter. This decision
will probably be made at a higher level in the organization, on the basis of the evaluation of concept
potential done within the team.
Having reduced the list to ideas that have real possibilities, a selection process should be used that
rates the concepts on all-important criteria. The specification document will provide not only the list
of criteria but also some guidance as to the importance of each item. This list should be used in
a two-step process; the first step will screen the concepts, and the second will select those to be
developed further.
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For the screening we will use a method called Pugh concept selection. Place a list of the criteria
to be used in the first column of a matrix, use the list of the remaining concepts as the headings for
the adjacent columns. A spreadsheet program will be very useful for this purpose. Choose one of the
concepts as a reference. This should be a well-understood idea, perhaps embodied in a current product,
yours or a competitor’s. Now compare each concept against the reference concept for each criterion
and score it with a plus sign if it does better and a minus sign if it is inferior. Use a zero if there is
no clear choice. When all the cells are full, add the number of pluses and subtract the number of
minuses to get the score for each idea. Many of the scores will be close to zero, and of course the
reference idea will get exactly zero. (See Table 1.2.)

TABLE 1.2 Partial Scoring of Concepts for External Fixation
Criteria Concept A Concept B Concept C Baseline concept
Weight + − + 0
Installation time − + + 0
Weakening + 0 − 0
Totals + # −# +# 0

At this point the number of concepts under consideration should be cut back to about 10 or 15.
The first criterion for the reduction is the score on the requirements sheet. Before discarding any of
the ideas, however, examine each one to determine why it has done so poorly. See if the idea may be
modified to increase its potential. If so, retain it for the next cycle.
With the field narrowed to a reasonable number of candidates, it is now possible to devote a little
effort to refining each of them, get to understand them better, and then construct a new matrix. This
time a weighting factor should be agreed to for each of the criteria. A range of 5 for very important
and 1 for minimally important will suffice. Now score each of the concepts on a basis of 1 to 10 on
each criterion and compute the sums of the weighted scores. This will allow the ideas to be ranked
and a selection made of the ideas to be seriously developed.
The major decision remaining here is how many concepts should be pursued in depth. This is
another judgment decision, to be guided by a number of factors: How much confidence is there in
the first two or three ideas? How long will it take to prove out the uncertainties? What resources are