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In other types of trade studies this may not be

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Unformatted text preview: the other. An explicit cost-effectiveness objective function is seldom available to help guide the selection decision, as any system engineer who has had to make a budget-induced system descope decision will attest. A second, and major, problem is that an expression or measurement method for system effectiveness may not be possible to construct, even though its underlying performance and technical attributes are easily quantified. These underlying attributes are often the same as the technical performance measures (TPMs) that are tracked during the product development process to gauge whether the system design will meet its performance requirements. In this case, system effectiveness may, at best, have several irreducible dimensions. What selection rule should be used has been the subject of many books and articles in the decision sciences —management science, operations research and economics. A number of selection rules are applicable to NASA trade studies. Which one should be used in a particular trade study depends on a number of factors: • • • • • The level of resolution in the system design The phase of the project life cycle Whether the project maintains an overall system effectiveness model How much less-quantifiable, subjective factors contribute to the selection Whether uncertainty is paramount, or can effectively be treated as a subordinate issue Whether the alternatives consist of a few qualitatively different architectures designs, or many similar ones that differ only in some quantitative dimen sions. This handbook can only suggest some selection rules for NASA trade studies, and some general conditions under which each is applicable; definitive guidance on which to use in each and every case has not been attempted. Table 3 first divides selection rules according to the importance of uncertainty in the trade study. This division is reflective of two different classes of decision problems —decisions to be made under conditions of certainty, and decisions to be made under conditions of uncertainty. Uncertainty is an inherent part of systems engineering, but the distinction may be best explained by reference to Figure 2, which is repeated here as Figure 24. In t...
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