Representing knowledge in rule-based systems [15]

RULE 1:
If the result of switching on the headlights is nothing happens or the result of trying the starter is nothing happens
Then the recommended action is recharge or replace the battery
RULE 2:
If the result of trying the starter is the car cranks normally and a gas smell is not present when trying the starter
Then the gas tank is empty with 90% confidence
RULE 3:
If the gas tank is empty
Then the recommended action is refuel the car
RULE 4:
If the result of trying the starter is the car cranks normally and a gas smell is present when trying the starter
Then the recommended action is wait 10 minutes, then restart flooded car

Each rule consists of an if part called the premise or antecedent (shown in blue) and a then part called the consequent or conclusion (shown in green). When the if part is true, the rule is said to fire and the then part is asserted -- it is considered to be a fact.

Rule results are often combined to reach a conclusion. The goal of the auto diagnosis is to find a recommended action: what to do to get the car started. Rule 3 tells what to do if the gas tank is empty and rule 2 could prove that the gas tank is empty. If rule 2 fires, rule 3 will also fire and provide a recommended course of action.

The consequent in rule 2 is asserted with 90% confidence. This means that if the rule's premise is true, we are only 90% certain that the car is out of gas. Our computer-based expert might be willing to accept this level of confidence to fire rule 3 and recommend an action.


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