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Unformatted text preview: above dichotomous categorizations stands above
the others. NASA Systems Engineering Handbook
Systems Analysis and Modeling Issues
Another taxonomy can be based on the degree
of analytic tractability. At one extreme on this scale, an
"analytic" model allows a closedform solution for a outcome variable of interest as a function of the model
inputs. At the other extreme, quantification of a outcome
variable of interest is at best ordinal, while in the middle
are many forms of mathematical simulation models.
Mathematical simulations are a particularly
useful type of model in trade studies. These kinds of
models have been successfully used in dealing
quantitatively with large complex systems problems in
manufacturing, transportation, and logistics. Simulation
models are used for these problems because it is not
possible to "solve" the system's equations analytically to
obtain a closedform solution, yet it is relatively easy to
obtain the desired results (usually the system's behavior
under different assumptions) using the sheer
computational power of current computers.
Linear, nonlinear, integer and dynamic program ming models are another important class of models in
trade studies because they can optimize an objective
function representing an important outcome variable (for
example, system effectiveness) for a whole class of
implied alternatives. Their power is best applied in
situations where the system's objective function and
constraints are well understood, and these constraints
can be written as a set of equalities and inequalities.
Pitfalls in Using Models. Models always embody assumptions about the real world they purport to represent,
and they always leave something out. Moreover, they
are usually capable of producing highly accurate results
only when they are addressing rigorously quantifiable
questions in which the "physics" is well understood as,
for example, a load dynamics analysis or a circuit
analysis.
In dealing with system issues at the top level,
however, this is seldom the case. There is often a
significant difference between the substantive system
costef...
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This document was uploaded on 02/26/2014 for the course E 515 at University of Louisiana at Lafayette.
 Spring '13
 Mr.Kau
 Systems Engineering, The American, ... ...

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