Conflict Set & Conflict Resolution

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Conflict Set

The set of possible rules which could be fired at any particular time is known as the conflict set. In a large system, there could be hundreds or thousands of rules within the conflict set at any given time. Because of this, the inference engine must use some form of conflict resolution to decide which rule from the conflict set to fire next.

Conflict Resolution

There are many different conflict resolution strategies which can be used, either separately or in combination. Some of the commonest ones are described briefly below:
  1. First-come first-served - the inference engine searches down from the top of the rule base until it finds a rule which can be fired. This is simple to implement, but means that the system designer has to plan carefully the order of the rules within the system.
  2. Most recent first - in this strategy, rules which use the most recent new information are fired before rules which use "old" information which has been in working memory for a while.
  3. Most specific first - in this strategy, the inference engine selects the most specific rules, that is, ones which have the greatest number of conditions which are true. So, a rule with 8 conditions all of which are true is fired before a more general rule with perhaps only two conditions which are true.
  4. Avoid repetition - the inference engine will avoid firing a rule which has been fired recently. This helps to avoid the system getting stuck in a loop.
  5. Setting priorities - the knowledge engineer can give higher priorities to certain rules, helping to steer the system towards the rules most likely to lead to a conclusion. However, this tends to undermine the "intelligence" of the system, and may prevent the system from discovering a surprising conclusion that the knowledge engineer had not expected.

Most forward chaining rule-based expert systems use a combination of these and other strategies to deal with conflict resolution and choose the most appropriate rule from the conflict set.

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