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Ontology and its research tool Protege

July 17, 2010

This post is contributed by Liyu Zheng, Ph.D student in industrial and systems engineering,Virginia Tech.

An ontology defines a common vocabulary for researchers who need to share information in a domain. In recent years, the development of ontologies has become one major focus of domain researchers. Ontologies is also in the common use in the development web techniques, for example, search engine index, social network site keyword index. One of greatest use of ontologies is for taxonomies categorizing the commercial products ( Amazon, ebay) and blog posts ( WordPress, Posterous).

Why ontologyies have been adopted in various business and scientific communities as a way to share, reuse and represent domain knowledge?  The motivations are:

  • Share common understanding  of the structure of information among people or software agents
  • Enable reuse of domain knowledge
  • Make domain assumptions explicit
  • Separate domain knowledge from the operational knowledge
  • Analyze domain knowledge

Basic Concepts of ontology include: classes (often also called concepts),  slots (also called roles or properties), facets (also called role restrictions) and instances. Ontology is a formal explicit description of concepts in a domain of discourse (classes), properties of each concept describing various features and attributes of concept (slots), and restrictions on slots (facets). An ontology together with a set of individual instances of classes constitutes a knowledge base.

Open source software has been used as research tools for ontology. In this article, I will focus on introducing some basis of Protégé, which includes Protégé-Frames and Protégé-OWL.

Protégé, from Standford, is a free, open-source platform with a growing user community. Protégé has a suite of tools to construct domain models and knowledge-based applications with ontologies. In its core, protégé implements a rich set of knowledge-modeling structures and actions that support the creation, visualization, and manipulation of ontologies in various representation formats. Protégé can be customized to provide domain-friendly support for creating knowledge models and entering data.

The Protégé platform supports two main ways of modeling ontologies:

The Protégé-Frames editor enables users to build and populate ontologies that are frame-based, in accordance with the Open Knowledge Base Connectivity protocol (OKBC). In this model, an ontology consists of a set of classes organized in subsumption hierarchy to represent a domain’s salient concepts, a set of slots associated to classes to describe their properties and relationships, and a set of instances of those classes-individual exemplars of the concepts that hold specific values of their properties.

Figure 1: screenshot example of Protégé-Frames

The Protégé-OWL editor allows users to build ontologes for the Semantic Web, in particular in W3C’s Web Ontology Language (OWL). An OWL ontology may include descriptions of classes, properties and their instances. Given such an ontology, the OWL formal semantics specifies how to derive its logical consequences, i.e. facts not literally present in the ontology, but entailed by the semantics. These entailments may be based on a single document or multiple distributed documents that have been combined using defined OWL mechanisms.

Figure 2: screenshot example of Protégé-OWL

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