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Rule Based Approach to knowledge representation | Advantages and Disadvantages

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rule based approachRule Based Approach with advantages and disadvantages
Fourth Semester | Tribhuvan University (TU)
Subject: Cognitive Science | BSc.CSIT

Rule Based Approach
Rule-based systems are used as a way to store and manipulate knowledge to interpret information in a useful way. In this approach, idea is to use production rules, sometimes called IF-THEN rules. The syntax structure is
IF <premise>THEN<action>
<premise>– is Boolean. The AND, and to a lesser degree OR and NOT, logical connectives are possible.
<action>– a series of statements


  • The rule premise can consist of a series of clauses and is sometimes referred to as the antecedent
  • The actions are sometimes referred to as the consequent

A typical rule-based system has four basic components:

  • A list of rules or rule base, which is a specific type of knowledge base.
  • An inference engine or semantic reasoner, which infers information or takes action based on the interaction of input and the rule base.
  • Temporary working memory.
  • A user interface or other connection to the outside world through which input and output signals are received and sent.

Working Memory contains facts about the world and can be observed directly or derived from a rule. It contains temporary knowledge – knowledge about this problem-solving session. It may be modified by the rules.
It is traditionally stored as <object, attribute, value> triplet.

Rule Base contains rules; each rule is a step in a problem solving process. Rules are persistent knowledge about the domain. The rules are typically only modified from the outside of the system, e.g. by an expert on the domain.
The syntax is a IF THEN format.
The conditions are matched to the working memory, and if they are fulfilled, the rule may be fired.

Actions can be:

  • Adding fact(s) to the working memory.
  • Removing fact(s) from the working memory
  • Modifying fact(s) in the working memory.

The Interpreter operates on a cycle:

  • Retrieval: Finds the rules that match the current Working Memory. These rules are the Conflict Set.
  • Refinement: Prunes, reorders and resolves conflicts in the Conflict Set.
  • Execution: Executes the actions of the rules in the Conflict Set. Applies the rule by performing the action.

Advantages of rule based approach:

  • Naturalness of Expression: Expert knowledge can often been seen naturally as rules of thumb.
  • Modularity: Rules are independent of each other – new rules can be added or revised later. Interpreter is independent from rules.
  • Restricted Syntax: Allows construction of rules and consistency checking by other programs. Allows (fairly easy) rephrasing to natural language.

Disadvantages (or limitations)

  • rule bases can be very large (thousands of rules)
  • rules may not reflect the actual decision making
  • the only structure in the KB is through the rule chaining

For example: “If the patient has stiff neck, high fever and an headache, check for Brain Meningitis”. Then it can be represented in rule based approach as:

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