Another desirable model characteristic is

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Unformatted text preview: effectiveness variable is not represented in the cost models. For example, if spacecraft reliability is a key variable in the system effectiveness equation, and if that reliability does not appear as a variable in the spacecraft cost model, then there is an important disconnect. This is because the models allow the spacecraft designer to believe it is possible to boost the effectiveness with increased reliability without paying any apparent cost penalty. When the models fail to treat such important interactions, the system engineer must ensure that others do not reach false conclusions regarding costs and effectiveness. Characteristics of a Good Model. In choosing a model (or models) for a trade study, it is important to recognize those characteristics that a good model has. This list includes: • • • • • Relevance to the trade study being performed Credibility in the eye of the decision maker Responsiveness Transparency User friendliness. Both relevance and credibility are crucial to the acceptance of a model for use in trade studies. Relevance is determined by how well a model addresses the substantive cost-effectiveness issues in the trade study. A model's credibility results from the logical consistency of its mathematical relationships, and a history of successful (i.e., cor- NASA Systems Engineering Handbook Systems Analysis and Modeling rect) predictions. A history of successful predictions lends credibility to a model, but full validation—proof that the model's prediction is in accord with reality—is very difficult to attain since observational evidence on those predictions is generally very scarce. While it is certainly advantageous to use tried-and-true models that are often left as the legacy of previous projects, this is not always possible. Systems that address new problems often require that new models be developed for their trade studies. In that case, full validation is out of the question, and the system engineer must be content with models that have logical consistency and some limited form of...
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This document was uploaded on 02/26/2014 for the course E 515 at University of Louisiana at Lafayette.

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