The second type of expert system - the model-based
system - uses a specific functional model to diagnose the observed symptoms
and devise a solution to the problem. The knowledge base is usually organized
around a functional or representative model of the system, but it is sometimes
preferable to use an actual physical model. This model now provides the
procedure with a focus of attention directed toward expected goals and guides
the process in determining the effects of system/equipment failure symptoms.
In the area of testability a number of detail models have been developed.
These include models such as: WSTA, STAT and STAMP which are addressed in more
detail in section 184.108.40.206.4.1 of this handbook.