As we have noted before, expert systems are used to solve real-life problems which do
not have a predefined solution to be found in the relevant literature. Generally, the knowledge
that is explicitly available on the subject is incomplete or uncertain. Nevertheless, a
human expert often can arrive at a sound solution to the given problem using such deficient
knowledge. Consequently, expert systems research aims at building systems capable of handling
incomplete and uncertain information as well as human experts are. Several models for
reasoning with uncertainty have been developed.