ARS uses its record of deductions to explain the derivation of facts to users, to find choices relevant to a contradiction, and to delineate facts believed to be true. Whenever an assumption is invalidated, the system scans forward from that choicepoint, marking all consequences of the discredited choice invalid. Evaluator, an expert system for the evaluation of structural designs for buildings, applies hierarchically organized design criteria to form a tree that includes inferred and implied nodes. The importance of any node signifies the extent to which a descendant can influence the values of its parent in the hierarchy. Since both text and graphics are important to design, a useful interface should include semantic as well as syntactic representation of design knowledge.
Drawings are a syntactical component in which entities have contextual dependence.
By contrast, reasoning functions of an expert system are semantic components that combine variables, value ranges, decisions, evaluations and justifications. Implementation Issues Research in the use of expert systems for design tasks has identified a range of implementation issues. Also of concern is the need for recovery methods if constraint conflicts go undetected. Regardless of the solution model applied, the effectiveness of AI design applications depends upon a minimum level of trust by professional users. If forced to use an aid which he does not trust, a decision maker may use any means available, even very demanding and time consuming ones, to direct the output of the aid toward his own decisions.