Simple Knowledge Organization System

Simple Knowledge Organization System (SKOS) is a W3C recommendation designed for representation of thesauri, classification schemes, taxonomies, subject-heading systems, or any other type of structured controlled vocabulary. SKOS is part of the Semantic Web family of standards built upon RDF and RDFS, and its main objective is to enable easy publication and use of such vocabularies as linked data.

DESIRE II project (1997–2000)
The most direct ancestor to SKOS was the RDF Thesaurus work undertaken in the second phase of the EU DESIRE project. Motivated by the need to improve the user interface and usability of multi-service browsing and searching, a basic RDF vocabulary for Thesauri was produced. As noted later in the SWAD-Europe workplan, the DESIRE work was adopted and further developed in the SOSIG and LIMBER projects. A version of the DESIRE/SOSIG implementation was described in W3C's QL'98 workshop, motivating early work on RDF rule and query languages: A Query and Inference Service for RDF.

LIMBER (1999-2001)
SKOS built upon the output of the Language Independent Metadata Browsing of European Resources (LIMBER) project funded by the European Community, and part of the Information Society Technologies programme. In the LIMBER project CCLRC further developed an RDF thesaurus interchange format which was demonstrated on the European Language Social Science Thesaurus (ELSST) at the UK Data Archive as a multilingual version of the English language Humanities and Social Science Electronic Thesaurus (HASSET) which was planned to be used by the Council of European Social Science Data Archives CESSDA.

SWAD-Europe (2002-2004)
SKOS as a distinct initiative began in the SWAD-Europe project, bringing together partners from both DESIRE, SOSIG (ILRT) and LIMBER (CCLRC) who had worked with earlier versions of the schema. It was developed in the Thesaurus Activity Work Package, in the Semantic Web Advanced Development for Europe (SWAD-Europe) project. SWAD-Europe was funded by the European Community, and part of the Information Society Technologies programme. The project was designed to support W3C's Semantic Web Activity through research, demonstrators and outreach efforts conducted by the five project partners, ERCIM, the ILRT at Bristol University, HP Labs, CCLRC and Stilo. The first release of SKOS Core and SKOS Mapping were published at the end of 2003, along with other deliverables on RDF encoding of multilingual thesauri and thesaurus mapping.

Semantic web activity (2004-2005)
Following the termination of SWAD-Europe, SKOS effort was supported by the W3C Semantic Web Activity in the framework of the Best Practice and Deployment Working Group. During this period, focus was put both on consolidation of SKOS Core, and development of practical guidelines for porting and publishing thesauri for the Semantic Web.

Later status and roadmap (2006-2008)
SKOS is a work in progress, and the main published documents — the SKOS Core Guide, the SKOS Core Vocabulary Specification, and the Quick Guide to Publishing a Thesaurus on the Semantic Web — have W3C Working Draft status. The main editors of SKOS are Alistair Miles and Dan Brickley.

The new Semantic Web Deployment Working Group, chartered for two years (May 2006 - April 2008), has put in its charter to push SKOS forward on the W3C Recommendation track. The roadmap projects SKOS as a Candidate Recommendation by the end of 2007, and as a Proposed Recommendation in the first quarter of 2008. The main issues to solve are determining its precise scope of use, and its articulation with other RDF languages and standards used in libraries (such as Dublin Core).

(2009-08-18)
On this date, W3C announced a new standard that builds a bridge between the world of knowledge organization systems - including thesauri, classifications, subject headings, taxonomies, and folksonomies - and the linked data community, bringing benefits to both. Libraries, museums, newspapers, government portals, enterprises, social networking applications, and other communities that manage large collections of books, historical artifacts, news reports, business glossaries, blog entries, and other items can now use SKOS to leverage the power of linked data.

Community and participation
All development work is carried out via the mailing list which is a completely open and publicly archived mailing list devoted to discussion of issues relating to knowledge organisation systems, information retrieval and the Semantic Web. Anyone may participate informally in the development of SKOS by joining the discussions on public-esw-thes@w3.org - informal participation is warmly welcomed. Anyone who works for a W3C member organisation may formally participate in the development process by joining the Semantic Web Deployment Working Group - this entitles individuals to edit specifications and to vote on publication decisions.

Components
SKOS is designed as a modular and extensible family of languages, and in a way that its use and implementation should be as simple as possible.

SKOS Core
SKOS Core defines the classes and properties sufficient to represent the common features found in a standard thesaurus. It is based on a concept-centric view of the vocabulary, where primitive objects are not terms, but abstract notions represented by terms. Each SKOS concept is defined as an RDF resource. Each concept can have RDF properties attached, including:
 * one or more preferred index terms (at most one in each natural language)
 * alternative terms or synonyms
 * definitions and notes, with specification of their language

Concepts can be organized in hierarchies using broader-narrower relationships, or linked by non-hierarchical (associative) relationships. Concepts can be gathered in concept schemes, to provide consistent and structured sets of concepts, representing whole or part of a controlled vocabulary.

These features represent the stable part of SKOS Core. Other elements of the vocabulary are still considered unstable.

SKOS Mapping
SKOS Mapping is intended to provide a vocabulary to express matching (exact or fuzzy) of concepts from one concept scheme to another. This part of SKOS has been developed in the SWAD-Europe project and currently has no official home. It is maintained informally by SKOS editors.

SKOS Extensions
SKOS Extensions are intended to provide ways to declare relationships between concepts with more specific semantics than the simple "broader-narrower", such as class-instance or partitive relationships. Like SKOS Mapping, this part is likely to stay in standby mode until SKOS Core is completed as a W3C Recommendation.

Metamodel
The SKOS metamodel is broadly compatible with the data model of ISO 25964-1 - Thesauri for Information Retrieval. This data model can be viewed and downloaded free of charge from the official website for ISO 25964.

Applications

 * Some important vocabularies have been migrated into SKOS format and are available in the public domain, including EuroVoc, AGROVOC and GEMET. Library of Congress Subject Headings (LCSH) also support the SKOS format.
 * SKOS has been used as the language for the thesauri used in the SWED Environmental Directory developed in the SWAD-Europe project framework.
 * A way to convert thesauri to SKOS, with examples including the MeSH thesaurus, has been outlined by the Vrije Universiteit Amsterdam.
 * Subject classification using DITA and SKOS has been developed by IBM.
 * SKOS is used to represent geographical feature types in the GeoNames ontology.

Tools

 * Mondeca's Intelligent Topic Manager (ITM) is a full-featured SKOS-compliant solution for managing taxonomies, thesauri, and other controlled vocabularies.
 * TemaTres Vocabulary Server is an open source web-based vocabulary server for managing controlled vocabularies, taxonomies and thesaurus. Tematres provides complete export of vocabularies into SKOS-core in addition to Zthes, TopicMaps, MADS, Dublin Core,VDEX, BS 8723, SiteMap, SQL and text.
 * ThManager is a Java open-source application for creating and visualizing SKOS vocabularies.
 * The W3C provides an experimental on-line validation service.
 * SKOS files can also be imported and edited in RDF-OWL editors such as Protégé or SWOOP developed by Maryland Information and Network Dynamics Lab Semantic Web Agents Project Mindswap.
 * SKOS synonyms can be transformed from WordNet RDF format using an XSLT stylesheet see W3C RDF
 * PoolParty is a commercial-quality thesaurus management system and a SKOS editor for the Semantic Web including text analysis functionalities and Linked Data capabilities.
 * qSKOS is an open-source tool for performing quality assessment of SKOS vocabularies by checking against a quality issue catalog.
 * SKOSEd is an open source plugin for the Protégé 4 OWL ontology editor that supports authoring SKOS vocabularies. SKOSEd has an accompanying SKOS API written in Java that can be used to build SKOS based applications.
 * Model Futures SKOS Exporter for Microsoft Excel allows simple vocabularies to be developed as indented Excel spreadsheets and exported as SKOS RDF. BETA VERSION.
 * Lexaurus is an enterprise thesaurus management system and multi-format editor. Its extensive API includes full revision management. SKOS is one of its many supported formats.
 * TopBraid Enterprise Vocabulary Net (EVN) is a web-based solution for simplified development and management of interconnected controlled vocabularies. It supports collaboration on defining and linking enterprise vocabularies, taxonomies, thesauri and ontologies used for information integration, customization and search.
 * Thesaurus Master, for creating, developing, and maintaining taxonomies and thesauri, is part of Access Innovations' Data Harmony knowledge management software line. It offers SKOS-compliant export.

Data
There are publicly available SKOS data sources.
 * SKOS DataZone wiki The W3C recommends to use this list of publicly available SKOS data sources in a wiki. Most data found there can be used for commercial and research applications.

SKOS and thesaurus standards
SKOS development has involved experts from both RDF and library community, and SKOS intends to allow easy migration of thesauri defined by standards such as NISO Z39.19 - 2005 or ISO 25964.

SKOS and other semantic web standards
SKOS is intended to provide a way to make a legacy of concept schemes available to Semantic Web applications, simpler than the more complex ontology language, OWL. OWL is intended to express complex conceptual structures, which can be used to generate rich metadata and support inference tools. However, constructing useful web ontologies is demanding in terms of expertise, effort, and cost. In many cases, this type of effort might be superfluous or unsuited to requirements, and SKOS might be a better choice. The extensibility of RDF makes possible further incorporation or extension of SKOS vocabularies into more complex vocabularies, including OWL ontologies.