AI - a Guide to Intelligent Systems.pdf - Member of EEPIS

BUY SMART: A FRAME-BASED EXPERT SYSTEM
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cases that are not represented in the rule base. However, it is not necessary
always to rely on a set of rules. We can use the WHEN NEEDED method to
establish the attribute value.
The WHEN NEEDED method shown in Figure 5.10 is attached to the attribute
Evaluation. The inference engine executes this method when it needs to
determine the value of Evaluation. When the WHEN NEEDED method is
executed, the attribute Evaluation receives the value Consult a superior.
How does the inference engine know where, and in what order, to obtain
the value of an attribute?
In our case, if the WHEN NEEDED method were executed first, the attribute
Evaluation would always receive the value Consult a superior, and no rules would
ever be fired. Thus, the inference engine has to obtain the value from the WHEN
NEEDED method only if it has not been determined from the rule base. In other
words, the search order for the attribute value has to be determined first. It can
be done, for example, by means of the SEARCH ORDER facet attached to an
attribute that tells the inference engine where, and in what order, to obtain the
value of this attribute.
In Level5 Object, a search order can be specified for every attribute, and in our
credit evaluation example, we set the search order for the Evaluation value to
RULES, WHEN NEEDED. It makes certain that the inference engine starts the
search from the rule base.
5.6 Buy Smart: a frame-based expert system
To illustrate the ideas discussed above, we consider a simple frame-based expert
system, Buy Smart, which advises property buyers.
We will review the main steps in developing frame-based systems, and show
how to use methods and demons to bring frames to life. To aid us in this effort
we will use the Level5 Object expert system shell.
Are there any differences between the main steps in building a rule-based
expert system and a frame-based one?
The basic steps are essentially the same. First, the knowledge engineer needs to
obtain a general understanding of the problem and the overall knowledge
structure. He or she then decides which expert system tool to use for developing
a prototype system. Then the knowledge engineer actually creates the knowledge
base and tests it by running a number of consultations. And finally, the expert
system is expanded, tested and revised until it does what the user wants it to do.
The principal difference between the design of a rule-based expert system and
a frame-based one lies in how the knowledge is viewed and represented in the
system.
In a rule-based system, a set of rules represents the domain knowledge useful
for problem solving. Each rule captures some heuristic of the problem, and each
new rule adds some new knowledge and thus makes the system smarter. The rulebased
system can easily be modified by changing, adding or subtracting rules.