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Ontology (PowerPoint)

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					Intelligent Systems

     Ontology
                 Ontology
• An ontology is an organisation of objects
  (individuals) into a class structure. It
  consists of three elements:

  – The individuals (objects or concepts)

  – Grouping of individuals into a class hierarchy

  – Properties associated with individuals/classes
         Individuals (objects)
• Objects or individuals (the term used in
  OWL) are the instances that form the
  bottom layer of the ontology;
• Examples include specific persons e.g.
  John, Ahmed, Mary, Fatma etc.
• It also include specific towns or cities e.g.
  Bolton, Manchester, Alexandria etc.
• And specific organisations e.g. Bolton
  University, British Aerospace etc.
         Classes (concepts)
• Such objects (individuals) belong to classes
  (concepts) e.g. persons, towns etc.
• Classes relate to one another in a
  hierarchical structure i.e. each class is
  either a subclass to something above it or is
  a superclass to something below it. This is
  referred to as subsumption.
• The topmost superclass is called „Thing‟
                 Classes
• The topmost superclass is called „Thing‟
• Sometimes subclasses and superclasses
  are referred to as child classes or parent
  classes respectively.
• In this sense, two classes being children to
  the same parent class are referred to as
  „sibling classes‟.
                      Classes
• A class hierarchy is also referred to as a
  taxonomy

• Naming conventions

   – the first character in a class name to be upper case


   – compound names should have no spaces, but start
     each name with an upper case character.
                   Classes
• Thinking of classes as sets of objects, they
  can either

  – overlap, referred to as intersection

  – Be clear of each other, referred to as disjoint

  – Or complete subsumption which would turn
    them into a subclass-superclass
    Properties (Relationships)

• Properties are indications of the identity
  and features of the class or object
  revealed through establishing a
  relationship between the respective
  individual or class and another individual
  or class or data item.
    Properties (Relationships)
• These are of three types:

  – Object properties

  – Data properties

  – Annotation properties
     Properties (Relationships)
• Object properties: connect objects to other
  objects e.g.
            John <hasMother> Jane

• Data properties: connect objects to some data-
  value e.g.
           John <hasAge> 25years

• Annotation properties: connect objects to some
  data string e.g.
            John <hasGraduated> “Bolton Uni”
    Properties (Relationships)
• Object properties can have:

  – Sub-property

  – Sibling-property

  – Inverse property
    Properties (Relationships)
Object properties can also have the
 following seven characteristics:
  – Functional property
  – Inverse functional property
  – Transitive property
  – Symmetric property
  – Anti-symmetric property
  – Reflexive property
  – Irreflexive property
    Properties (Relationships)
• Properties are identified by the domain
  through which they initiate and the range
  to which they connect

• Consider the relation:
          Ahmed <hasTeacher> Ali
and Ahmed is student while Ali is a teacher,
  then [Student] is the domain while
  [Teacher] is the range.
    Properties (Relationships)
• Properties are identified by the domain
  through which they initiate and the range
  to which they connect

• Consider the relation:
          Ahmed <hasTeacher> Ali
and Ahmed is student while Ali is a teacher,
  then [Student] is the domain while
  [Teacher] is the range.
    Properties (Relationships)

• A restriction describes a class of
  individuals based on the relationships that
  members of the class participate in. In
  other words a restriction is a kind of class,
  in the same way that a named class is a
  kind of class.
    Properties (Relationships)
• Types of restrictions:

  – Quantifier restrictions
     • Existential restrictions
     • Universal restrictions


  – Cardinality restrictions
  – hasValue restrictions
    Properties (Relationships)
Based on the properties associated with
  classes, two types of classes can be
  identified:
• Asserted classes: these are the named
  classes defined by the user;

• Inferred classes: these are the
  anonymous classes defined by properties
  and inferred by the reasonser
             The reasoner
• The reasoner checks for the consistency
  of the class hierarchy.

• To do this, a fictitious subclass is added as
  being child of two disjoint classes.

• Such a class is called probe class
   Primitive & defined classes
• A class that only has necessary
  conditions is known as a Primitive Class.
• A class that has at least one set of
  necessary and sufficient conditions is
  known as a Defined Class.
• Necessary conditions are simply called
  Superclasses in Prot´eg´e 4. Necessary
  and sufficient condition are called
  Equivalent classes.