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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
• The level of resolution in the system design The
phase of the project life cycle
Whether the project maintains an overall system
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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