| AI Book
Mycin Expert System
A Ruby Implementation
What is Mycin
Mycin is an expert system that can be programmed with knowledge database for different domains (e.g. medical diagnosis). It asks questions, reasons, gives explanations on why it asked the question, and deals with uncertainties. See the example below.
This is a Ruby rewrite of the Lisp code given in the book:
Peter Norvig: "Paradigms of Artificial Intelligence Programming. Case Studies in Common Lisp" Morgan Kaufmann Publishers, San Francisco. 1992 ISBN 1-55860-191-0 Original LISP source code is available HERE
Excerpts from the book:
MYCIN lead to the development of the EMYCIN expert-system shell... EMYCIN is a backward-chaining rule interpreter that has much in common with Prolog. However, there are four important differences. First, and most importantly, EMYCIN deals with uncertainty. Instead of insisting that all predications be true or false, EMYCIN associates *certainty factor* with each predication. Second, EMYCIN caches the results of its computation so that they not be duplicated. Third, EMYCIN provides an easy way for the system to ask the user for information. Fourth, it provides explanations of its behavior. This can be summed up in the equation:EMYCIN = Prolog + uncertainty + caching + questions + explanationsThe MYCIN expert system was one of the earliest and remain one of the best known. It was written by Dr. Edward Shortliffe in 1974 as an experiment in medical diagnosis.
A typical rule:
param context operation value ----- ------- --------- ----- (defrule 52 <--- rule number if (site culture is blood ) (gram organism is neg ) <--- premise/condition (morphl organism is rod ) (burn patient is serious) then 0.4 <--- confidence factor (cf) (identity organism is pseudomonas ) <--- conclusion )
There is nothing to install really. Install Ruby first, then
This is a result of a test run, user's entries are in bold. The example is from the book, although some questions are out of order (probably due to random order when reading a Hash).
>test_mycin.rb ------- patient-1 ------- Patient's name: HELP Type one of the following: ? - to see possible answers for this parameter rule - to show current rule why - to see why this question is asked help - to see this list xxx - (for some specific xxx) if there is a definite answer xxx .5 yyy .4 - if there are several answers with different certainty factors Patient's name: SYLVIA_FISHER Sex: FEMALE Age: 27 ------- culture-1 ------- From what site was specimen CULTURE-1 taken? BLOOD How many days ago was this culture (CULTURE-1) obtained? 3 ------- organism-1 ------- Enter the identity (genus) of ORGANISM-1? UNKNOWN The gram strain of ORGANISM-1? ? Must be one of: acid-fast, pos, neg The gram strain of ORGANISM-1? NEG Is ORGANISM-1 a rod or coccus? ROD Is PATIENT-1 a burn patient? If so, mild or serious? WHY [Why is the value of burn being asked for?] It is known that: The site of the culture is blood The gram of the organism is neg The morphology of the organism is rod Therefore, Rule 52: If The burn of the patient is serious Then with certainty of 0.4 The identity of the organism is pseudomonas Is PATIENT-1 a burn patient? If so, mild or serious? SERIOUS What is the aerobicity of ORGANISM-1? AEROBIC Is PATIENT-1 a compromised host? YES Findings for ORGANISM-1 for these goals: identity IDENTITY: ENTERO 0.8, PSEUDOMONAS 0.76 Is there another organism?YES ------- organism-2 ------- Enter the identity (genus) of ORGANISM-2? UNKNOWN The gram strain of ORGANISM-2? NEG 0.8 POS 0.2 Is ORGANISM-2 a rod or coccus? ROD What is the aerobicity of ORGANISM-2? ANAEROBIC Findings for ORGANISM-2 for these goals: identity IDENTITY: BACTEROIDES 0.72, PSEUDOMONAS 0.6464 Is there another organism?NO Is there another culture?NO Is there another patient?NO