687.1 Lecture 6 1Rule-Based Systems6.871-- Lecture 66.871 - Lecture 6 1Outline MYCIN– Introduction»Task» Sample Interaction» Reasons for Success– How It Worked» Knowledge Representation» Architecture and Control Structure» Inexact Inference» Explanation» Interaction Help– Reasons for Success Revisited– Contributions Prospector “Rules Glasses”6.871 - Lecture 6 2MYCIN What was the task?– Medical diagnosis and therapy selection for bacterial infections of the blood Why this domain?– Overuse of antibiotics6.871 - Lecture 6 4Why This Domain Overuse of antibiotics Irrational use of antibiotics Maldistribution of expertise Domain is “small” and “isolated”6.871 - Lecture 6 5---------PATIENT-248----------1) Patient’s name: (first-last)** CR2) Sex:** MALE3) Age:** 524) Have you been able to obtain positive microbiological information about a possible infection of C.R.?** YES----------INFECTION--------------5) What is the infection?** ENDARTERITISSample Interaction: Clinical Data6.871 - Lecture 6 6The first significant organism from this blood culture (CULTURE-1)will be referred to as:--------------ORGANISM-1-----------------6) Enter the identity of ORGANISM-1** UNKNOWN7) Is ORGANISM-1 a rod or coccus (etc.):** ROD8) The gram stain of ORGANISM-1** GRAMNEG9) HAVE THERE BEEN positive cultures yielding organismsabout which you will NOT be seeking advice?** YESSample Interaction (cont’d)687.1 Lecture 6 26.871 - Lecture 6 79) Enter the identity of ORGANISM-2:** SALMONELLA (.4) E.COLI (.3)Sample Interaction (cont’d)6.871 - Lecture 6 8Is the patient’s illness with ORGANISM-1 a hospital-acquired infection?** WHY[i.e. WHY is it important to determine whether or not the infection with ORGANISM-1 was acquired while the patient was hospitalized][2.0] This will aid in determining the identity of ORGANISM-1.It has already been established that[2.1] the morphology of ORGANISM-1 is rod[2.2] the gram stain of ORGANISM-1 is gramneg[2.3] the aerobicity of ORGANISM-1 is faculTherefore, if[2.4] the infection with ORGANISM-1 was acquired while the patient was hospitalizedthenthere is weakly suggestive evidence (.2) that the identity of ORGANISM-1 is pseudomonas [RULE050]Sample Interaction: Explanation6.871 - Lecture 6 9INFECTION-1 is ENDARTERITIS with BACTEREMIA<Item 1> E. COLI <Item 2> SALMONELLA (species unknown)<Item 3> KLEBSIELLA-PNEUMONIAE<Item 4> PSEUDOMONAS-AERUGINOSA <Item 5> ENTEROBACTER<Item 6> PROTEUS-NON-MIRABILUSSample Interaction: Diagnosis6.871 - Lecture 6 10[REC 1] My preferred therapy recommendation is as follows:In order to cover for Items <1 3 4 5 6 >:Give: GENTAMICINDose:128 mg (1.7. Mg/kg) q8h IV (or IM) for 10 daysComments: Modify dose in renal failureIn order to cover for Item <2>:Give: CHLORAMPHENICOLDose:563 mg (7.5 mg.kg) q6h for 14 daysComments: Monitor patient’s white countDo you wish to see the next choice therapy?** NOSample Interaction: Therapy6.871 - Lecture 6 11How It Worked: RepresentationRule 27If: 1)the gram stain of the organism is gram negative, and2)the morphology of the organism is rod, and3)the aerobicity of the organism is anaerobic,Then:There is suggestive evidence (.7) that the identity of the organism is Bacteriodes.6.871 - Lecture 6 14INFERENCE ENGINEKNOWLEDGEBASEArchitecture687.1 Lecture 6 36.871 - Lecture 6 15IDENTITY = BacteroidesGRAM MORPHOLOGY AEROBICITYRule 27Control Structure…Rule 1546.871 - Lecture 6 16Control Structure What kind of search?– Backward-chaining, depth-first With what variations?– Generalized subgoalsE.g. “determine identity”, not “is identity = E.coli”– Every rule relevant to goal is explored– Inexact inference6.871 - Lecture 6 17R1: A and B X (.4)R2: C and D X (.3)A and B and C and D ?Inexact Inference Desiderata:– Commutative–Asymptotic– Balances positive and negative evidence6.871 - Lecture 6 18R1: A and B X (.4)R2: C and D X (.3)A and B and C and D ?Inexact Inference “increase certainty .3 more from current value”X(.4 + .3(1 - .4)) = X(.58)current value remaining uncertaintynew valueA and B X (.4)C and D:6.871 - Lecture 6 19R1: A (.5) and B (.6) X (.4)R2: C (.4) and D (.7) X (.3)A and B and C and D ?Inexact Inference AND: min (A and B) = .5min (C and D) = .4R1: .5 * .4 = .2R2: .4 * .3 = .12Combining: .2 + .12( 1 - .2) = .30current valueremaining uncertaintynew value6.871 - Lecture 6 20** HOW DID YOU DECIDE THAT ORGANISM-1 WAS AN E.COLI?I used rule084 to conclude that the identity of ORGANISM-1 is e.coli. This gave a cumulative c.f. of (.51).I used rule003 to conclude that the identity of ORGANISM-1 is e.coli. This gave a cumulative c.f. of (.43).Explanation687.1 Lecture 6 46.871 - Lecture 6 21** WHY DIDN’T YOU THINK THAT ORGANISM-1 WAS NEISSERIA?The following rules could have been used to determine that the identity of ORGANISM-1 is neisseria: 9.However, none of these succeeded in the context of ORGANISM-1. If you would like an explanation for why any of these rules failed, please enter their numbers:** 9Clause 2 of rule009 [“the morphology of the organism is coccus”] was already known to be false for ORGANISM-1, so the rule was never tried.Explanation6.871 - Lecture 6 22Successful? Appropriate task: Narrow domain, heuristic knowledge, articulate experts, etc. The match of knowledge and representation:– Knowledge about of how the body works, vs.– Knowledge about how to diagnose it Answer was a ranked list, not a single result Perhaps most important:Having the right abstractions, the right language6.871 - Lecture 6 23The Right AbstractionIf …the organism is gram-negative, andthe portal of entryis skin-wound,Then the organism is likely to be … It’s an abstraction It’s the right abstraction for this task Where did it come from?6.871 - Lecture 6 24What ought a rule to be?Answer:An independentsingle-stepinferentialempirical association.6.871 - Lecture 6 25What ought a rule to be? Independent– Does it make sense in isolation? Single step: mental hygiene Inferential: focus on the knowledge level Empirical association– Association: symptom & disease– Empirical: noticed but not understood– Logic-like, but:»Inexact» Not truth-preserving6.871 - Lecture 6 26What Ought a Rule to Be?IF There is pressure on project leader, Respondent is R&D manager,No increase in growth
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