Have the failure modes been identified?

224 CHAPTER 8 Concept Evaluation and Selection Note that if it is impossible to make a comparison to a design requirement, more information must be developed. This may require more analysis, further experimentation, or just better visualization. It may even be necessary to refine the design, through the methods to be described in Chaps. 9–11 and then return to make the comparison. Note that the frailty in doing this step is the topic of Sections 8.6 and 8.7. In using the Decision Matrix there are two possible types of comparisons. The first type is absolute in that each alternative concept is directly i.e., absolutely compared with some target set by a criterion. The second type of comparison is relative in that alternative concepts are compared with each other using measures defined by the criteria. In choosing to use a datum the comparison is relative. However, many people use the method for absolute comparisons. Absolute com- parisons are possible only when there is a target. Relative comparisons can be made only when there is more than one option. Step 6: Compute the Satisfaction and Decide What to Do Next. After a concept is compared with the datum for each criterion, four scores are generated: the number of plus scores, the number of minus scores, the overall total, and the weighted total. The overall total is the difference between the number of plus scores and the number of minus scores. This is an estimate of the decision-makers’ satisfaction with the alternative. The weighted total can also be computed. This is the sum of each score multiplied by the importance weighting, in which an S counts as 0, a + as +1, and a – as –1. Both the weighted and the unweighted scores must not be treated as absolute measures of the concept’s value; they are for guidance only. The scores can be interpreted in a number of ways: ■ If a concept or group of similar concepts has a good overall total score or a high + total score, it is important to notice what strengths they exhibit, that is, which criteria they meet better than the datum. Likewise, groupings of scores will show which requirements are especially hard to meet. ■ If most concepts get the same score on a certain criterion, examine that criterion closely. It may be necessary to develop more knowledge in the area of the criterion in order to generate better concepts. Or it may be that the criterion is ambiguous, is interpreted differently by different members of the team, or is unevenly interpreted from concept to concept. If the criterion has a low importance weighting, then do not spend much time clarifying it. However, if it is an important criterion, effort is needed either to generate better concepts or to clarify the criterion. ■ To learn even more, redo the comparisons, with the highest-scoring concept used as the new datum. This iteration should be redone until a clearly “best” concept or concepts emerge. After each team member has completed this procedure, the entire team should compare each member’s individual results. The results can vary widely, since neither the concepts nor the requirements may be refined. Discussion among the members of the group should result in a few concepts to refine. If it does