When the technical analysis indicates that you have some potentially viable designs, the
best one available must be selected for detailed design, prototyping, and testing. The
selection process usually involves a comparative analysis of the available design solutions.
A decision matrix sometimes helps to identify the best solution by forcing you to
consider a variety of factors in a systematic way. A decision matrix for our better grass
shortener is shown in Figure 1-2. Each design occupies a row in the matrix. The columns
are assigned categories in which the designs are to be judged, such as cost, ease of
use, efficiency, performance, reliability, and any others you deem appropriate to the particular
problem. Each category is then assigned a weighting factor, which measures its
relative importance. For example, reliability may be a more important criterion to the
user than cost, or vice versa. You as the design engineer have to exercise your judgment
as to the selection and weighting of these categories. The body of the matrix is then filled
with numbers which rank each design on a convenient scale, such as 1 to 10, in each of
the categories. Note that this is ultimately a subjective ranking on your part. You must
examine the designs and decide on a score for each. The scores are then multiplied by
the weighting factors (which are usually chosen so as to sum to a convenient number
such as 1) and the products summed for each design. The weighted scores then give a
ranking of designs. Be cautious in applying these results. Remember the source and subjectivity
of your scores and the weighting factors! There is a temptation to put more faith
in these results than is justified. After all, they look impressive! They can even be taken
out to several decimal places! (But they shouldn't be.) The real value of a decision
matrix is that it breaks the problem into more tractable pieces and forces you to think
about the relative value of each design in many categories. You can then make a more
informed decision as to the "best" design.