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Some of these are shown in table 5 unfortunately none

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Unformatted text preview: lidity of CERs based on weight, a common indevariable in many models, in light of advances in electronic packaging and composite materials. Objections to using statistical CERs also include problems of input accuracy, low statistical significance due to limited data points, ignoring the statistical confidence bands, and, lastly, biases in the underlying data. tance, to obtain an estimate of cost. Similarly, software CERs have been developed as well, relying on judgments about source lines of code and other factors to obtain development costs. (See sidebar on statistical CERs.) Another type of parametric model relies on accepted relationships. One common example can be found in the application of logistics relationships to the estimation of repair costs and initial and recurring spares costs. The validity of these cost estimates also depends on the quality of the input parameters. The principal advantages of parametric cost models are that the results are reproducible, are more easily documented than other methods, and often can be produced with the least amount of time and effort. This makes a properly constructed performance-based parametric cost model useful in early trade studies. Analogy is another way of estimating costs. When a new system or component has functional and performance characteristics similar to an existing one whose cost is known, the known cost can be adjusted to reflect engineering judgments about differences. Grass-roots (or "bottom-up") estimates are the result of rolling up the costs estimated by each organization performing work described in the WBS. Properly done, grass-roots estimates can be quite accurate, but each time a `'what if" question is raised, a new estimate needs to be made. Each change of assumptions voids at least part of the old estimate. Because the process of obtaining grassroots estimates is typically time-consuming and labor-intensive, the number of such estimates that can be prepared during trade studies is in reality severely limited. Whatever technique is used, the direct cost of each hardware and software element often needs to be &...
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