Metadata principle 3. Good metadata uses authority control and content standards such as controlled vocabularies that are in line with user expectations to describe the content of objects and collocate related objects.

Attributes of distributed objects should be expressed according to standard controlled vocabularies when possible. These include, but are not limited to, personal names, corporate names, place names, subjects, and genre headings. Classification schemes, a form of controlled vocabulary that groups related resources into a hierarchical structure, can be useful in providing online subject access.

As with metadata schemes, there are many published thesauri, taxonomies, and authority files, and there is no "one size fits all" solution. The choice of vocabularies to use will depend to some extent on factors such as the metadata scheme chosen and the resources of the institution. Authors and other untrained metadata creators can not generally be counted on to use controlled vocabularies successfully unless the authority list is very short and simply organized.

Other important factors include:

• The anticipated users of the digital collection. Will they be adults or children, specialists or generalists? What languages do they speak? What other resources are they likely to use, and what vocabularies are employed in those?
• Tools to support the use of the vocabulary. Is there an online thesaurus? Can it be incorporated into the collection's search system? Are there cross-references and related terms?
• Maintenance. New terms come into use, and old terms become archaic or obsolete. Who maintains the vocabulary, and how are updates issued?

Whatever vocabularies are chosen, their use should be documented and guidelines should be provided to help metadata creators select terms consistently.

Controlled vocabularies, thesauri and classification systems available in the WWW lists several dozen web-accessible controlled vocabularies by subject area. http://www.lub.lu.se/metadata/subject-help.html

The High Level Thesaurus Project (HILT) is a clearinghouse of information about controlled vocabularies, including related resources, projects, and an alphabetical list of thesauri. http://hilt.cdlr.strath.ac.uk/Sources/index.html

The Getty Vocabulary Program builds, maintains, and disseminates several thesauri for the visual arts and architecture:

• Art & Architecture Thesaurus (AAT). http://www.getty.edu/research/conducting_research/vocabularies/aat/
• Union List of Artist Names (ULAN)
  http://www.getty.edu/research/conducting_research/vocabularies/ulan/
• Getty Thesaurus of Geographic Names (TGN). http://www.getty.edu/research/conducting_research/vocabularies/tgn/

Some other controlled vocabularies are:

• Library of Congress Authorities files. Search subject, name, title, and name/title headings from LC's online catalog. http://authorities.loc.gov/
• Medical Subject Heading List. http://www.nlm.nih.gov/mesh/
• Thesaurus for Graphic Materials I: Subject Terms. http://lcweb.loc.gov/rr/print/tgm1/
• Thesaurus for Graphic Materials II: Genre and Physical Characteristics Terms. http://lcweb.loc.gov/rr/print/tgm2/
• The Geographic Names Information System. http://geonames.usgs.gov/

Classification systems available on the Web include

• Dewey Decimal Classification. [Subscription required for access.] http://connexion.oclc.org/
• Library of Congress Classification. [Subscription required for access.] http://classweb.loc.gov/

Source: adapted from 'A Framework of Guidance for Building Good Digital Collections', published by the National Information Standards Office, 2nd edition, 2004.