Chapter 7

INFERENCE AND CONTROL

Inference engine performs 2 major tasks:

    We shall look at  Inference and Control

  7.1 INFERENCE

     7.1.1  MODUS PONENS

     *most common inference strategy
     *simple ,reaoning based on it is easily understood.
 
     The rule states that when A is known to be true and if a rule states " If A then B "
     it is valid to conclude that B is true.

     7.1.2  MODUS TOLLENS

    When B is false rule If A, then B
                                    then A is false.

        E.g:
          Rule  :       IF Ahmet's CAR IS DIRTY
                     THEN Ahmet HAS BEEN DRIVING OUTSIDE ANKARA

  Given fact : Ahmet has not been outside Ankara.
  New rule   : Ahmet car is not dirty.

   This conclusion seems quite obvious but cannot be reached by most expert systems.
   Because they use modus ponens for deriving new facts from rules.

 7.2 REASONING ABOUT UNCERTAINTY

 7.3 FORWARD CHAINING

     Forward Chaining (Example 2)

 Rule 1
     IF the car overheats , THEN the car will stall.
 Rule 2
      IF the car stalls
THEN it will cost me money
 AND  I will be late getting home
 
  Now, the question is
         How do you arrive at conclusion that this situation will cost money and cause you to be late ?
   The condition  that triggers the chain of events is the car overheating

 7.4 BACKWARD CHAINING

Rule 1
       IF the car is not tuned AND the battery is weak
 THEN not enough current will reach the starter.
Rule 2
      IF not enough current reaches the starter
THEN the car will not start.
 
Given facts:
     The car is not tuned
     The battery is weak.
  Now, the question is
         How  would you arrive at the conditions that have resulted in the car failing to start?
 

      In such a situation backward chaining might be more cost-effective.With this inference method the system starts with what it wants to prove, e.g.,that situation Z exists, and only executes rules that are relavent to establishing it.Figure  following shows how bacward chaining would work using the rules from the forward chaining example.
 
      In step 1 the system is told to establish (if it can) that situation Z exists,It first checks the data base for Z,and when that fails, searches for rules that conclude Z,i.e., have Z on the right side of the arrow.It finds the rule F?B->Z, and decides that it must establish F and B in order to conclude Z.

      In step 2 the system tries to establish F, first checking the data base and then finding a rule that concludes F. From this rule, C?D->F, the system decides it must establish C and D to conclude F.

      In steps 3 through 5 the system finds C in the data base but decides it must establish A before it can conclude D. It then finds A in the data base.

      In steps 6 through 8 the system executes the third rule to establish D, then executes the second rule to establish the original goal, Z.The infenece chain created here is identical to the one created by forward chaining. The difference in two approaches hinges on the method in which data and rules are searched.
 

 7.5 MIXED CHAINING

  R₄. IF A and G then B          }  R₄-R₈
  R₅. IF B and D then H          }  are forward chaining
  R₆. IF G and D then E          }
  R₇. IF A and B then D          }
  R₈. IF A and C then G          }
 

  1) Mixed Chaining with priority to backward chaining
      only resort to forward chaining when unable to backward chaining
        Assume working memory has {A,C} ?goal to determine K.

    B    B                         B
    R₁,R₃,R₈,R₄,R₇,R₅,R₆,R₂

 2) Mixed Chaining with priority to forward chaining
     some rule -set and goal and facts
 
    R₈,R₄,R₇,R₅,R₆,R₂(B)
 
   Result: 9 steps v.s +steps

  CONTROL
 
   There are two problems addressed by the inference engine:

    1 ) It must have a way to decide where to start.Rules and facts reside in a static knowledge base. There must be a way for the reasining process to begin.
 
    2 ) The inference engine must resolve conflicts that occur when alternative links of reasining emerge,The system may reach a point where there are more than a few rules ready to fire. The inference engine must choose which rule to examine next.

  CONFLICT RESOLUTION STRATEGIES (A PARTIAL LIST )

•   Refractoriness- Once a given rule fires then that same rule will be disallowed for subsequent firing.( Avoid applying a rule more than once for the same situation )
•   Recency          -  Rules that apply to the most recently working elements are chosen in preference to thosewhich apply to older working elements.

      x occurs twice at time t₁ has taken value a and at time t₂ obtained the value b.

• Specificity     - Rules which have more conditions on the left hand side are chosen in preference to those with fewer conditions.

    A rulebase can easily be enlarged by adding new, more specific rules to it without worrying too much about older ones, because more specific Production Rules will prevail over more general ones.

    For example, a person encountering a friend in the street will not be inclined to think this person is a mammal , but instead think of the person by name , just applying the most specific knowledge.
 
    Give priority to rules with more specific antecedents.

A general rule : If B And D Then P And Q And L.
A specific rule : If A and B And C And D Then J And K And L.

Other possible conflict resolution strategies :
 

• Rank rules according to "firing desirability " a priority.
• Give priority to those rules which are least recently used.
• Choose the rules that have the largest number of consequents, with the assumption that more is better.