Your body while holding a 5-kg weight straight out in front of you with your

10.3 The Goals of Performance Evaluation 281 Figure 10.1 Fox Float RP23 used on the Marin Mount Vision. Reprinted with permission of Fox Racing Shox. evolution of the product, the refinement of the functional decomposition also aids in the evaluation of potential failure modes covered in Chap. 11. Finally, tracking the evolution of function means continuously updating the flow models of energy, information, and materials. It is these flows that determine the performance of the product. As the product matures, the intended function and actual behavior merge and so what was, in conceptual design, concern for “the desired” now turns to measuring “the reality.” 10.3 THE GOALS OF PERFORMANCE EVALUATION In Chap. 6, we developed a set of engineering requirements based on the needs of the customer. For each of these requirements, a specific target was set. The goal now is to evaluate the product design relative to these targets. Since the targets are represented as numerical values, the evaluation can only occur after the product is refined to the point that numerical engineering measures can be made. In Chap. 8, the concepts developed were not yet refined enough to compare with the targets and were thus compared with measures that are more abstract. Evaluation can now be based on comparison with the engineering requirements. Beyond comparison to the requirements, effective evaluation procedures should 282 CHAPTER 10 Product Evaluation for Performance and the Effects of Variation Evaluation always requires a clear head and twice the time you estimated. clearly show what should be altered patched in order to make deficient products meet the requirements, and they should demonstrate the product’s insensitivity to variation in the manufacturing processes, aging, and operating environment. Restated, the evaluation of product performance must support these factors: 1. Evaluation must result in numerical measures of the product for comparison with the engineering requirement targets developed during problem under- standing. These measurements must be of sufficient accuracy and precision for the comparison to be valid. 2. Evaluation should give some indication of which features of the product to modify, and by how much, in order to bring the performance on target. 3. Evaluation procedures must include the influence of variations due to manu- facturing, aging, and environmental changes. Insensitivity to these “noises” while meeting the engineering requirement targets results in a robust, quality product. Where traditionally engineering evaluation has focused on only the first of these three points, this chapter covers all three. Much emphasis is placed on the third point, the consideration of variation because of its direct relationship with product quality. This chapter is built around Fig. 10.2, the P-diagram. This diagram will be referenced and added to throughout this chapter. In the P-diagram, the letter “P” stands for either product or process and can represent the entire product or some system, subsystem, or process within it. The product or process being evaluated is dependent on the values of many parameters. These parameters may be physical dimensions, material properties, forces from other systems, or forces and motions from humans controlling the system. They may be the temperature of Product or process Parameters Quality measures Change values or redesign Acceptable Target Figure 10.2 The basic P-diagram.