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There has been some research effort into and development of a number of techniques to support V&V of expert system, and in identifying the most appropriate phases in the development to which they may be applied. Thus, opportunities to perform verification and validation occur at various stages during the development of the expert system. In the waterfall approach to software development, V&V can be carried out at the end of each phase of development. The equivalent waterfall model for expert systems (Figure 11) is similar in a way but presents a number of particularities (Coenen and Bench-Capon, 1993). These include:
More or less serious criticisms of the traditional waterfall model have led to alternative models such as rapid prototyping and spiral modeling, which have been developed to improve or replace the waterfall model. These models have been adopted by developers of expert systems as well. The software life cycle for spiral modeling is somewhat similar to the prototype model. In the analysis stage, the software development team specifies in the requirements documents of the software project the validation procedures, risk identification, a number of KB requirements, constraints on the computer memory required by the KB, schedule, resources, and an overall test strategy, including a verification and validation test plan. The attempts to define life-cycle models for expert system construction has led to the possibility of coupling the V&V activities to the specific stages in the development. Thus, V&V techniques are not stand-alone, but part of this methodology. However, important to notice is that the focus of V&V is not the same at each stage since the objectives of each are stage different. An example of such a mapping between different development stages and the focus of V&V in each of them is summarized in Table 3. Such a mapping gives guidance to what V&V can be done when. Essentially, V&V is spread over four stages: (1) requirement analysis, (2) knowledge acquisition, (3) knowledge specification and refinement and (4) implementation.
3.4. VERIFICATION AND VALIDATION SYSTEMSTo test for the anomalies and errors identified in previous chapters, many verification and validation systems have been built over the last decade. In the following, we will survey some of them, mentioning that the survey is necessarily incomplete, and it is based on depth rather than breadth. Our primary motivation in choosing these particular systems is that they provide a good cross-section of available verification and validation types of methods and tools. These systems along with others can be found in Vermesan and Bench-Capon (1995) and Preece et al. (1992), where pointers to the developers of the systems are also provided.
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