An expert system is a form of artificial intelligence. It uses human knowledge gathered from experts to form a knowledge base consisting of 1 to 500 rules. The system queries the user about the case and uses the rules to arrive at a diagnosis. At any stage, the user can ask the system why it is making a particular query or how it reached a particular decision. Expert systems are superior to human thought processes in that they preserve the expert's knowledge and can be more accurate than any one person. However, their expertise is limited to a specific area. The National Center for the Analysis of Violent Crime is currently developing an expert system called PROFILER to perform criminal personality profiling. The two main steps in developing such a system are the purchase of a shell -- a prewritten program -- and the creation of a knowledge base and set of rules. Baltimore County (Md.) is currently using an expert system to solve burglaries. 12 references.