Expert system

Expert system

In artificial intelligence, an expert system is a computer system that emulates the decision-making ability of a human expert.[1] Expert systems are designed to solve complex problems by reasoning about knowledge, like an expert, and not by following the procedure of a developer as is the case in conventional programming.[2][3][4] The first expert systems were created in the 1970s and then proliferated in the 1980s.[5] Expert systems were among the first truly successful forms of AI software.[6][7][8][9][10][11]

An expert system has a unique structure, different from traditional programs. It is divided into two parts, one fixed, independent of the expert system: the inference engine, and one variable: the knowledge base. To run an expert system, the engine reasons about the knowledge base like a human[12]. In the 80's a third part appeared: a dialog interface to communicate with users.[13] This ability to conduct a conversation with users was later called "conversational".[14][15]


Software architecture

The rule base or knowledge base

In expert system technology, the knowledge base is expressed with natural language rules IF ... THEN ... For examples :

  • "IF it is living THEN it is mortal"
  • "IF his age = known THEN his year of birth = date of today - his age in years"
  • "IF the identity of the germ is not known with certainty AND the germ is gram-positive AND the morphology of the organism is "rod" AND the germ is aerobic THEN there is a strong probability (0.8) that the germ is of type enterobacteriacae"[16]

This formulation has the advantage of speaking in everyday language which is very rare in computer science (a classic program is coded). Rules express the knowledge to be exploited by the expert system. There exists other formulations of rules, which are not in everyday language, understandable only to computer scientists. Each rule style is adapted to an engine style. The whole problem of expert systems is to collect this knowledge, usually unconscious, from the experts. There are methods but almost all are usable only by computer scientists.

The inference engine

The inference engine is a computer program designed to produce a reasoning on rules. In order to produce a reasoning, it is based on logic. There are several kinds of logic: propositional logic, predicates of order 1 or more, epistemic logic, modal logic, temporal logic, fuzzy logic, etc. Except propositional logic, all are complex and can only be understood by mathematicians, logicians or computer scientists. Propositional logic is the basic human logic, that expressed in the syllogism. The expert system that uses that logic are also called zeroth-order expert system. With logic, the engine is able to generate new information from the knowledge contained in the rule base and data to be processed.

The engine has two ways to run: batch or conversational. In batch, expert system has all the necessary data to process from the beginning. For the user, the program works as a classical program: he provides data and receives results immediately. Reasoning is invisible. The conversational becomes necessary when the developer knows he can't ask the user all the necessary data at the start, the problem being too complex. The software must "invent" the way to solve the problem, request the user missing data, gradually, approaching the goal as quickly as possible. The result gives the impression of a dialogue led by an expert. To guide a dialogue, the engine may have several levels of sophistication : "forward chaining", "backward chaining" and "mixed chaining". Forward chaining is the questioning of an expert who has no idea of the solution and investigates progressively (e.g. fault diagnosis). In backward chaining, the engine has an idea of the target (e.g. is it okay or not? Or: there is danger but what is the level?). It starts from the goal in hopes of finding the solution as soon as possible. In mixed chaining the engine has an idea of the goal but it is not enough: it deduces in forward chaining from previous user responses all that is possible before asking the next question. So, quite often, he deduces the answer to the next question before asking it.

A strong interest in using logic is that this kind of software is able to give to user clear explanation of what it is doing (the "Why?") and what it has deduced (the "How?" ). Better yet, thanks to logic the most sophisticated expert system are able to detect contradictions[17] into user informations or in the knowledge and can explain them clearly, revealing at the same time the expert knowledge and his way of thinking.


Expert systems offer many advantages for users when compared to traditional programs because they operate like a human brain.


Quick availability and opportunity to program itself

As the rule base is in everyday language (the engine is untouchable), expert system can be written much faster than a conventional program, by users or experts, bypassing professional developers and avoiding the need to explain the subject.

Ability to exploit a considerable amount of knowledge

The expert system uses a rule base, unlike conventional programs, which means that the volume of knowledge to program is not a major concern. Whether the rule base has 10 rules or 10 000, the engine operation is the same.


The reliability of an expert system is the same as the reliability of a database, i.e. good, higher than that of a classical program.


Evolving an expert system is to add, modify or delete rules. Since the rules are written in plain language, it is easy to identify those to be removed or modified.


The engines that are run by a true logic are able to explain to the user in plain language why they ask a question and how they arrived at each deduction. In doing so, they show knowledge of the expert contained in the expert system. So, user can learn this knowledge in its context. Moreover, they can communicate their deductions step by step. So, the user has information about their problem even before the final answer of the expert system.

Preservation and improvement of knowledge

Valuable knowledge can disappear with the death, resignation or retirement of an expert. Recorded in an expert system, it becomes eternal. To develop an expert system is to interview an expert and make the system aware of their knowledge. In doing so, it reflects and enhances it.

New areas neglected by conventional computing

Automating a vast knowledge, the developer may meet a classic problem: "combinatorial explosion" that greatly complicates his work and results in a complex and time consuming program. The reasoning expert system does not encounter that problem since the engine automatically loads combinatorics between rules. This ability can address areas where combinatorics are enormous: highly interactive or conversational applications, fault diagnosis, decision support in complex systems, educational software, logic simulation of machines or systems, constantly changing software.


The expert system has a major flaw which explains its low success although the principle has existed for 70 years: knowledge collection and interpretation into rules, the knowledge engineering. Most developers have no method to perform this task. They work "manually" what opens to many possibilities for errors. Expert knowledge is not well understood, there is a lack of rules, rules are contradictory, some are poorly written and unusable. Worse, they most often use an engine unable to reasoning. Result: the expert system works badly and the project is abandoned.[18] Correct development methodology can mitigate these problems. There exists software to interview the expert step by step which automatically write the rules and simultaneously run the expert system before his eyes, performing a consistency of the rules control[19][20][21]. So expert and users can check the quality of the software before it is finished.

Many expert systems are also penalized by the logic used. Most of the logics operate on "variables" facts, i.e. whose value changes several times during one reasoning, considered as a property belonging to a more powerful logic. Exactly like in classical computing, a way of programming where developers are in fact comfortable. This is the case of Mycin, Dendral, fuzzy logic, predicate logic (Prolog), symbolic logic, mathematical logic, etc.. Propositional logic uses only not variable facts[22]. It turns out that in human mind, the facts used must remain invariable as long as the brain reasons on them. This makes possible detection of contradictions and production of explanations, two ways of controlling consistency of the knowledge[23][24]. That is why expert systems using variable facts, more understandable for IT developers so the most numerous, are less easy to develop, less clear to users, less reliable and why they don't produce explanation or contradiction detection.

Application field

Expert systems address areas where combinatorics is enormous:

They can also be used in software engineering for rapid prototyping applications (RAD). Indeed, the expert system quickly developed in front of the expert shows him if the future application should be programmed.

Indeed, any program contains expert knowledge and classic programming always begins with an expert interview. A program written in the form of expert system receives all the specific benefits of expert system, among others things it can be developed by anyone without computer training and without programming languages. But this solution has a defect: expert system runs slower than a traditional program because he consistently "thinks" when in fact a classic software just follows paths traced by the programmer.

Examples of applications

Expert systems are designed to facilitate tasks in the fields of accounting, medicine, process control, financial service, production, human resources, among others. Typically, the problem area is complex enough that a more simple traditional algorithm cannot provide a proper solution. The foundation of a successful expert system depends on a series of technical procedures and development that may be designed by technicians and related experts. As such, expert systems do not typically provide a definitive answer, but provide probabilistic recommendations.

An example of the application of expert systems in the financial field is expert systems for mortgages. Loan departments are interested in expert systems for mortgages because of the growing cost of labour, which makes the handling and acceptance of relatively small loans less profitable. They also see a possibility for standardized, efficient handling of mortgage loan by applying expert systems, appreciating that for the acceptance of mortgages there are hard and fast rules which do not always exist with other types of loans. Another common application in the financial area for expert systems are in trading recommendations in various marketplaces. These markets involve numerous variables and human emotions which may be impossible to deterministically characterize, thus expert systems based on the rules of thumb from experts and simulation data are used. Expert system of this type can range from ones providing regional retail recommendations, like Wishabi, to ones used to assist monetary decisions by financial institutions and governments.

Another 1970s and 1980s application of expert systems, which we today would simply call AI, was in computer games. For example, the computer baseball games Earl Weaver Baseball and Tony La Russa Baseball each had highly detailed simulations of the game strategies of those two baseball managers. When a human played the game against the computer, the computer queried the Earl Weaver or Tony La Russa Expert System for a decision on what strategy to follow. Even those choices where some randomness was part of the natural system (such as when to throw a surprise pitch-out to try to trick a runner trying to steal a base) were decided based on probabilities supplied by Weaver or La Russa. Today we would simply say that "the game's AI provided the opposing manager's strategy".

Knowledge engineering

The building, maintaining and development of expert systems is known as knowledge engineering.[25] Knowledge engineering is a "discipline that involves integrating knowledge into computer systems in order to solve complex problems normally requiring a high level of human expertise".[26]

There are generally three individuals having an interaction in an expert system. Primary among these is the end-user, the individual who uses the system for its problem solving assistance. In the construction and maintenance of the system there are two other roles: the problem domain expert who builds the system and supplies the knowledge base, and a knowledge engineer who assists the experts in determining the representation of their knowledge, enters this knowledge into an explanation module and who defines the inference technique required to solve the problem. Usually the knowledge engineer will represent the problem solving activity in the form of rules. When these rules are created from domain expertise, the knowledge base stores the rules of the expert system.


Expert systems were introduced by researchers in the Stanford Heuristic Programming Project, including the "father of expert systems" Edward Feigenbaum, with the Dendral and Mycin systems. Principal contributors to the technology were Bruce Buchanan, Edward Shortliffe, Randall Davis, William vanMelle, Carli Scott and others at Stanford. Expert systems were among the first truly successful forms of AI software.[6][7][8][9][10][11]

Research is also very active in France, where researchers focus on the automation of reasoning and logic engines. The French Prolog computer language, designed in 1972, marks a real advance over expert systems like Dendral or Mycin: it is a shell[27], that's to say a software structure ready to receive any expert system and to run it. It integrates an engine using First-Order logic, with rules and facts. It's a tool for mass production of expert systems and was the first operational declarative language[28], later becoming the best selling IA language in the world[29]. However Prolog is not particularly user friendly and is an order of logic away from human logic [30][31][32].

In the 1980s, expert systems proliferated as they were recognized as a practical tool for solving real-world problems. Universities offered expert system courses and two thirds of the Fortune 1000 companies applied the technology in daily business activities.[5][33] Interest was international with the Fifth Generation Computer Systems project in Japan and increased research funding in Europe. Growth in the field continued into the 1990s.

The development of expert systems was aided by the development of the symbolic processing languages Lisp and Prolog. To avoid re-inventing the wheel, expert system shells were created that had more specialized features for building large expert systems.[34]

In 1981 the first IBM PC was introduced, with MS-DOS operating system. Its low price started to multiply users and opened a new market for computing and expert systems. In the 80's the image of IA was very good and people believed it would succeed within a short time[15]. Many companies began to market expert systems shells from universities, renamed "generators" because they added to the shell a tool for writing rules in plain language and thus, theoretically, allowed to write expert systems without a programming language nor any other software[16]. The best known: Guru (USA) inspired by Mycin[17][18], Personal Consultant Plus (USA)[19][20], Nexpert Object (developed by Neuron Data, company founded in California by three French)[21][22], Genesia (developed by French public company Electricité de France and marketed by Steria)[23], VP Expert (USA)[24]. But eventually the tools were only used in research projects. They did not penetrate the business market, showing that AI technology was not mature.

In 1986, a new expert system generator for PCs appeared on the market, derived from the French academic research: Intelligence Service[35][36], sold by GSI-TECSI software company. This software showed a radical innovation: it used propositional logic ("Zeroth order logic") to execute expert systems, reasoning on a knowledge base written with everyday language rules, producing explanations and detecting logic contradictions between the facts. It was the first tool showing the AI ​​defined by Edward Feigenbaum in his book about the Japanese Fifth Generation, Artificial Intelligence and Japan's Computer Challenge to the World (1983): "The machines will have reasoning power: they will automatically engineer vast amounts of knowledge to serve whatever purpose humans propose, from medical diagnosis to product design, from management decisions to education", "The reasoning animal has, perhaps inevitably, fashioned the reasoning machine", "the reasoning power of these machines matches or exceeds the reasoning power of the humans who instructed them and, in some cases, the reasoning power of any human performing such tasks". Intelligence Service was in fact "Pandora" (1985)[37], a software developed for their thesis by two academic students of Jean-Louis Laurière[38], one of the most famous and prolific French AI researcher[39]. Unfortunately, as this software was not developed by his own IT developers, GSI-TECSI was unable to make it evolve. Sales became scarce and marketing stopped after a few years.


  1. ^ Jackson, Peter (1998), Introduction To Expert Systems (3 ed.), Addison Wesley, p. 2, ISBN 978-0-201-87686-4 
  2. ^ Nwigbo Stella and Agbo Okechuku Chuks, School of Science Education, Expert system: a catalyst in educational development in Nigeria: "The ability of this system to explain the reasoning process through back-traces (...) provides an additional feature that conventional programming does not handle"
  3. ^ Regina Barzilay, Daryl McCullough, Owen Rambow, Jonathan DeCristofaro, Tanya Korelsky, Benoit Lavoie: "A new approach to expert system explanations"
  4. ^ Conventional programming
  5. ^ a b Cornelius T. Leondes (2002). Expert systems: the technology of knowledge management and decision making for the 21st century. pp. 1-22. ISBN 978-0-12-443880-4. 
  6. ^ a b ACM 1998, I.2.1
  7. ^ a b Russell & Norvig 2003, pp. 22−24
  8. ^ a b Luger & Stubblefield 2004, pp. 227–331
  9. ^ a b Nilsson 1998, chpt. 17.4
  10. ^ a b McCorduck 2004, pp. 327–335, 434–435
  11. ^ a b Crevier 1993, pp. 145–62, 197−203
  12. ^ Nwigbo Stella and Agbo Okechuku Chuks, School of Science Education, Expert system: a catalyst in educational development in Nigeria: "Knowledge-based systems collect the small fragments of human know-how into a knowledge-base which is used to reason through a problem, using the knowledge that is appropriated"
  13. ^ Koch, C. G.; Isle, B. A.; Butler, A. W. (1988). "Intelligent user interface for expert systems applied to power plant maintenance and troubleshooting". IEEE Transactions on Energy Conversion 3: 71. doi:10.1109/60.4202.  edit
  14. ^ McTear, M. F. (2002). "Spoken dialogue technology: Enabling the conversational user interface". ACM Computing Surveys 34: 90. doi:10.1145/505282.505285.  edit
  15. ^ Lowgren, J. (1992). "The Ignatius environment: Supporting the design and development of expert-system user interfaces". IEEE Expert 7 (4): 49–57. doi:10.1109/64.153464.  edit
  16. ^ Mycin rule
  17. ^ Nabil Arman, Polytechnic University of Palestine, January 2007, Fault Detection in Dynamic Rule Bases Using Spanning Trees and Disjoin Sets: ""
  18. ^ Kenneth Laudon, Jane Laudon, Eric Fimbel, "Management Information Systems: Managing the Digital Firm", Business & Economics, 2010 edition, chapter 11-3.5: The implementation of a large number of expert systems requires the deployment of considerable development efforts, lengthy and expensive. Hiring and training a larger number of experts may be less expensive than building an expert system .(...) Some expert systems, particularly the largest, are so complex that over years, the costs of curative and adaptive maintenance become as high as the cost of development.
  19. ^ Systèmes Experts, April 15, 1990, Miao, authentic expert system generator of fault diagnosis: "MIAO can explain, again in [plain] language, all of his logical approach: why he is asking such a question and how it came to such a conclusion. And that because he is constantly reasoning and not because an IT developer programmed in advance all the possible explanations."
  20. ^ Olivier Rafal, Le Monde Informatique, Programming for all (T.Rex generator): "This software allows to develop a conversational application (...) leading to a self-learning" (i.e. thanks to the automatic explanations)
  21. ^ French Technology Survey, MAIEUTICA, An Expert System Generator which writes its own rules, July 1991: "checking the coherence of the knowledge", "it can detect contradictions", "it react appropriately to changes of minds"
  22. ^ RGU: School of Computing, More Complex Inference: "propositional logic, where variables are not allowed".
  23. ^ Ong K. and Lee R.M, Texas University-Austin, A logic model for maintaining consistency of bureaucratic policies, 1993: "Inconsistencies can be detected if any of the integrity constraints is proven false, and an explanation can be provided based on the proof tree. A more general inference mechanism is presented based on the theory of abduction for checking potential inconsistency of policies"
  24. ^ Carl G. Hempel and Paul Oppenheim, Philosophy of Science, Studies in the Logic of Explanation, 1948: "The sentences constituting the explanans must be true"
  25. ^ Kendal, S.L.; Creen, M. (2007), An introduction to knowledge engineering, London: Springer, ISBN 978-1-84628-475-5, OCLC 70987401 
  26. ^ Feigenbaum, Edward A.; McCorduck, Pamela (1983), The fifth generation (1st ed.), Reading, MA: Addison-Wesley, ISBN 978-0-201-11519-2, OCLC 9324691 
  27. ^ George F. Luger and William A. Stubblefield, Benjamin/Cummings Publishers, Rule Based Expert System Shell: example of code using the Prolog rule based expert system shell
  28. ^ A. MICHIELS, Université de Liège, Belgique: "PROLOG, the first declarative language
  29. ^ Carnegie Mellon University's AI Web Site: "Prolog was the most popular AI language in Japan and probably in Europe"
  30. ^ Ivana Berković, Biljana Radulović and Petar Hotomski, University of Novi Sad, 2007, Extensions of Deductive Concept in Logic Programing and Some Applications: "the defects of PROLOG-system: the expansion concerning Horn clauses, escaping negation treatment as definite failure"
  31. ^ "Software developed in Prolog has been criticized for having a high performance penalty compared to conventional programming languages"
  32. ^ Dr. Nikolai Bezroukov, Softpanorama: "I think that most people exposed to Prolog remember strongly the initial disappointment. Language was/is so hyped but all you can see initially are pretty trivial examples that are solved by complex, obscure notation that lacks real expressive power: some of simple examples can be expressed no less concisely is many other languages"
  33. ^ Durkin, J. Expert Systems: Catalog of Applications. Intelligent Computer Systems, Inc., Akron, OH, 1993.
  34. ^ page 21. Giarratano & Riley, 3rd ed.
  35. ^ Flamant B. and Girard G., GSI-TECSI, Intelligence Service: build your own expert system : "Intelligence Service is a development environment for expert systems that requires no experience of classic programming that offers to everyone the opportunity to develop its own expert system"
  36. ^ Bertrand Savatier, Le Monde Informatique, November 23, 1987: "Expert systems accessible to all"
  37. ^ Jean-Philippe de Lespinay, Science et Vie, "From total zero to Zero Plus [logic]", May 1991
  38. ^ Death of Jean-Louis Laurière
  39. ^ Journey "In honor of Jean-Louis Laurière", Université Pierre et Marie Curie in Paris (March 22, 2006)



  • Darlington, Keith (2000). The Essence of Expert Systems. Pearson Education. ISBN 978-0-13-022774-4. 
  • Ignizio, James (1991). Introduction to Expert Systems. McGraw-Hill Companies. ISBN 978-0-07-909785-9. 
  • Giarratano, Joseph C. and Riley, Gary; Gary Riley (2005). Expert Systems, Principles and Programming. Course Technology Ptr. ISBN 978-0-534-38447-0. 
  • Jackson, Peter (1998). Introduction to Expert Systems. Addison Wesley. ISBN 978-0-201-87686-4. 
  • Walker, Adrian et al. (1990). Knowledge Systems and Prolog. Addison-Wesley. ISBN 978-0-201-52424-6. 

History of AI


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