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Unformatted text preview: to the cost analyst at each point in the project life cycle. Table 4 shows this dependence. Parametric (or "top-down") cost models are most useful when only a few key variables are known or can be estimated. The most common example of a parametric model is the statistical Cost Estimating Relationship (CER). A single equation (or set of equations) is derived from a set of historical data relating one or more of a system's characteristics to its cost using well-established statistical methods. A number of statistical CERs have been developed to estimate a spacecraft's hardware acquisition cost. These typically use an estimate of its weight and other characteristics, such as design complexity and inheri- NASA Systems Engineering Handbook Systems Analysis and Modeling Issues Statistical Cost Estimating Relationships: Example and Pitfalls One model familiar to most cost analysts is the historically based CER. In its usual form, this model is a linear expression with cost (the dependent variable) as a function of one or more descriptive characteristics. The coefficients of the linear expression are estimated by fitting historical data from previously completed projects of a similar nature using statistical regression techniques. This type of model is analytic and deterministic. An example of this type of model for estimating the first unit cost, C, of a space-qualified Earth-orbiting receiver/exciter is: In C = 3.822 + 1.110 In W + 0.436 In z where W is the receiver/exciter's weight, and z is the number of receiver boxes; In is the natural logarithm function. (Source: U.S. Air Force Systems Command-Space Division, Unmanned Space Vehicle Cost Model, Seventh Edition, August 1994.) CERs are used extensively in advanced technology systems, and have been challenged on both theoretical and practical grounds. One challenge can be mounted on the basis of the assumption of an unchanging relationship between cost and the independent variables. Others have questioned the va...
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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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