No specification can be regarded as complete without at least one scenario for each mode of operation identified. A scenario is a description of the environment (both dynamic and static aspects) in which the system being specified will be ultimately embedded. Specification scenarios enable the required performance and reliability of the proposed system to be evaluated and/or verified prior to any investment in design. Thus, for example, the need to develop (and become snared by) prototype designs, in order to develop confidence in a specification, is eliminated. In addition, comprehensive concept acceptance testing can be carried out long before any committment to consequential design and in a manner that would not be affordable - or even physically possible - with the real system.
For any project, the importance of using a model to verify the functional aspects of the Statement of Requirements (SOR) cannot be over-estimated. The SOR drives all project activities and it dictates the content of all technical documents - such as the System Design, the Build Specification and the Support Plan. Project Managers rightly apply engineering disciplines (e.g. design engineering, production engineering, test engineering and logistics engineering) to assure themselves of the quality of these technical documents, so they surely ought to apply an equally rigorous discipline to develop a similar level of quality assurance in the source material for all technical documents - namely the SOR.
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