Page 142 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

There are more quantitative methods for screening alternatives. One set of methods known as concept
screening and concept scoring [4] will be briefly summarized here. More details can be found in Ulrich
and Eppinger [4]. These methods are useful in that they allow subjective assessments to be quantified
systematically for comparison purposes.

In concept screening, a selection matrix is prepared by listing a set of criteria to be used to evaluate the
alternatives. Then one alternative is chosen as a reference alternative. This should be an alternative with
which the team doing the evaluation is most familiar, perhaps an industry standard. All criteria for the
reference standard are assigned a value of zero, meaning “same as.” The criteria for all other alternatives
are assigned values of +, meaning “better than”; zero; or –, meaning “worse than.” Then the number of
“worse thans” is subtracted from the number of “better thans.” The net score for each alternative provides
a relative ranking. Some type of reflection is needed at this stage to determine whether the results make
sense and whether each criterion was assigned a reasonable value. The number of alternatives is now
reduced, though it is up to those involved to determine how many alternatives survive to the next step. An

example of concept screening is shown in Table 4.2. Here, alternative 5 is chosen as the reference
alternative. It is observed that alternatives with equal scores are assigned the same rank. To proceed to
the next step, we will assume that only four alternatives—those with positive scores—remain in the
selection process.

Table 4.2 Example of Concept Screening

In concept scoring, the same matrix is used, but only on those alternatives that have survived the concept
screening process. The results are now more quantitative. Each criterion is now assigned a relative
weight, which reflects the team’s judgment as to its relative importance. A reference alternative is chosen.
Then, for each alternative, each criterion is assigned a value from 1 to 5, where 1 = much worse than
reference, 2 = worse than reference, 3 = same as reference, 4 = better than reference, and 5 = much better
than reference. The score is calculated for each alternative by weighting the evaluations using the relative
weights. Once again, some degree of reflection on the result is needed because this is a subjective
process, particularly the assigning of relative weights. The best alternative is the one with the highest

score. Because there is a large degree of subjectivity here, care should be exercised when differentiating
between alternatives with close scores. Table 4.3 illustrates concept scoring for the four alternatives
chosen during concept screening. Based on this method, alternative 1 is chosen for further study, although
alternative 7 is close. Once again, small differences in total score may not be significant. Also,
information obtained during product development may change the relative weights and/or individual
scores sufficiently so that the total score changes enough and alternative 7 is actually the best choice.

Table 4.3 Example of Concept Scoring