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.
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