Are the critical parameters identified?

8.5 The Decision Matrix—Pugh’s Method

223 Step 2: Select the Alternatives to Be Compared. The alternatives to be com- pared are the different ideas developed during concept generation. It is important that all the concepts to be compared be at the same level of abstraction and in the same language. This means it is best to represent all the concepts in the same way. Generally, a simple sketch is best. In making the sketches, ensure that knowledge about the functionality, structure, technologies needed, and manufacturability is at a comparable level in every figure. Step 3: Choose the Criteria for Comparison. First, it is necessary to know the basis on which the alternatives are to be compared with each other. Using the QFD method in Chap. 6, an effort was made to develop a full set of customer requirements for a design. These were then used to generate a set of engineer- ing requirements and targets that will be used to ensure that the resulting prod- uct will meet the customer requirements. However, the concepts developed in Chap. 7 might not be refined enough to compare with the engineering targets for evaluation. If they are not, we have a mismatch in the level of abstraction and use of the engineering targets must wait until the concept is refined to the point that actual measurements can be made on the product designs. Usually the basis for comparing the design concepts is a mix of customer requirements and engineering specifications, matched to the level of fidelity of the alternatives. If the customers’ requirements have not been developed, then the first step should be to develop criteria for comparison. The methods discussed in Chap. 6 should help with this task. Additionally, the technology readiness measures can also help with evaluation here. This is especially true if the alternatives are dependent on new technologies. Step 4: Develop Relative Importance Weightings. In step 3 of the QFD method Section 6.4 there is a discussion of how to capture the relative importance of the criteria. The methods developed there can be used here to indicate which of the criteria are more important and which are less important. It is often worthwhile to measure the relative importance for different groups of customers, as discussed in Section 6.4. Step 5: Evaluate Alternatives. By this time in the design process, every de- signer has a favorite alternative; one that he or she thinks is the best of the concepts that have yet to be developed. This concept is used as a datum, all other designs being compared with it as measured by each of the customer requirements. If the problem is for the redesign of an existing product, then the existing product, abstracted to the same level as the concepts, can be used as the datum. For each comparison, the concept being evaluated is judged either better than, about the same as, or worse than the datum. If it is better than the datum, the concept is given a + score. If it is judged to be about the same as the datum or if there is some ambivalence, an S “same” is used. If the concept does not meet the criterion as well as the datum does, it is given a – score. If the Decision Matrix is on a spreadsheet use +1, 0, –1 for scoring. 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