MYCIN

• provide consultative advice on diagnosis and therapy for infectious diseases.

• MYCIN informs itself about particular cases by requesting information about a patient's symptoms, general condition, history, and the easy, quickly obtainable laboratory tests.

• Medical knowledge is encoded as rules.

Example:

RULE 050

PREMISE: (AND (SAME CNTXT INFECT PRIMARY-BACTEREMIA)
(MEMBF CNTXT SITE STERILESITES)
(SAME CNTXT PORTAL GI))
ACTION (CONCLUDE CNTXT IDENT BACTEROIDES TALLY .7)

MYCIN English translation:

IF 1) the infection is primary-bacteremia
2) the site of the culture is one of the sterile site, and
3) the suspected portal of entry of the organism is t
the gastrointestinal tract
THEN there is suggestive evidence (.7) that the identity of the
organism is bacteroides.

• MYCIN can translate its rules (in LISP form) to readable English form.

• Contains 450 rules.

• The premise of each rule is a Boolean combination of one or more clauses, each is constructed from a predicate function with an associative triple û (attribute, object, value) û as its argument.

• A premise clause
  <predicate function> <object> <attribute> <value>

• e.g. the site of the culture is one of the sterile sites:

(MEMBF	CNTXT	SITE	STERILESITES)
¡	¡	¡	¡
Predicate	Object	Attribute	Value

• A rule premise is always a conjunction of clauses

The Inference Engine

• rules are invoked in a simple backward-chaining fashion that results in an exhaustive depth-first search of an AND-OR goal tree.

• The subgoal that is set up is a generalized form of the original goal. I.e. the subgoal is always of the form:
  Determine the value of <attribute>
  rather than
  Determine whether the <attribute> is equal to <value>

• Example: For Rule 50, the subgoal set up will be
  Determine the type of infection for the clause
  (the infection is primary-bacteremia)

• MYCIN uses meta-rules
  e.g.
  IF 1) the infection is a pelvic-abscess
  2) there are rules that mention in their premises Enterobacteriaceae
  3) there are rules that mention in the premise gram positive rods
  THEN there is suggestive evidence that the rules dealing with
  Enterobacteriaceae should be evoked before those dealing with
  gram positive rods.
  
  
  

• Since the rules are inexact (leading to conclusions of less than total certainty), it is a wisely conservative strategy to continue to collect all evidence about the subgoal from other applicable rules even if one rule succeeds.

• MYCIN considers all the possibilities every time.
  
  
  

• MYCIN use the strategy: always attempting to deduce the value of a subgoal and ask the user only when deduction fails.

• This ensures a minimum no. of questions.

• Lazy evaluation of premises is used.

Uncertainty in MYCIN

• Rules contain certainty factors, cf.

  • they make inexact inferences on a confidence scale of -0.1 to 1.0.

  • 1.0 represents complete confidence that it is true.

  • -1.0 represents complete confidence that it is false.

• The Cfs are measurements of the association between the premise and action clauses of each rules.
  
  
  

• When a production rule succeeds because its premise clauses are true in the current context, the Cfs of the component clauses that indicate how strongly each clause is believed are combined, the resulting CF is used to modify the CF specified in the action clause.

• MYCIN's reasoning with uncertainty is based on probability theory.

• When value lies between -0.2 and 0.2, it is interpreted as unknown.

Sample Consultation

@mycin

MYCIN 26-March ...

Instruction? (Y or N)

N

26-March 10:42

---PATIENT-1 ----

Mycin has created an object (PATIENT-1) in its dynamic knowledge base or working memory. Certain attributes are immediately requested and are necessary for every patient. These make up the first four questions...

1. Patient name:
   Alan Apt

2. Age?
   34

3. Sex
   Why
   [1.0] The sex of the patient is one of the four standard parameters that identify a patient. This may be useful later...
   [back to question 3]
   Male

4. Race?
   Caucasian

5. Are there any cultures for Alan Apt that may be related to the present illness, and from which organisms have been grown successfully in the lab?
   No.

6. Are there any pending cultures for Alan Apt?
   Yes.
   ....

36. Has Alan Apt had neurosurgery?
    Why
    [6.0] This will aid in determining the organisms (other than those seen on cultures or smears) that might be causing the infection.
    It has already been established that
    [6.1] the culture that requires therapy is meningitis, and
    [6.2] organisms were not seen on the stain of the pending csf culture
    (CULTURE-1), and
    [6.3] the type of infection is bacterial
    Therefore, if
    [6.4] Alan Apt has had neurosurgery, and
    [6.5] the time since the neurosurgical operation is less than two months, and
    [6.6] the type of neurosurgery is not ventricular-urethral-shunt,
    then
    there is evidence that the organisms that might be causing the infection are
    staphylococcus-coag-pos (.75), staphylococcus-coag-neg (.5) e.coli(.4),
    klebsiella....
    [RULE 411]
    ...