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3. BOXES-AND-ARROWS APPROACHESOne of the earliest examples in the literature is that of Buchanan et al. (1983). As the book in which this paper appears is from the dawn of the commercial uptake of expert systems, it has exerted a considerable influence. Figure 2 reproduces Buchanan et al.'s approach. Gradually this has been extended into what has come to be called the "Expert Systems Development Life Cycle"3 or ESDLC, which by minor or major variations have been modified conventional systems development approaches. The dominant variant is the waterfall approach, though sometimes (see, e.g., Khan, 1992) references are made to prototyping and even spiral-model approaches derived from the work of Boehm (1988).
As has already been said in the introduction, it is impossible to track all the proposals that have been made over the last 15 years in this area. In the U.S., the major effort seems to have been in the area of mapping the expert systems life cycle to existing standards for conventional software engineering (e.g., DoD-Std-2167A), which resembles the approach described in Section 5 of linking with existing methodologies for conventional systems. To summarize these efforts it seems sufficient to take one of the most recent books about building KBSs (Awad, 1996) as an example4. The life cycle given in this book is depicted in Figure 3.
Though the world view behind approaches like the one in Figure 3 is rarely made explicit, a few of them can be inferred from the properties of Figure 3:
Variants of this approach mostly will exhibit the following modifications:
Given the abundance of schemes of the type depicted in Figure 3 and the intractability of their possible development into the other two classes, it is impossible to make any definite statement about what they have achieved. From the superficial review of the literature reported in Section 6 it appears that boxes-and-arrows approaches have been used most frequently for building KBSs.
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