6.3 Using Ontologies for Formulating Queries 141
records, available separately. There are 23,887 cross-references that assist in
finding the most appropriate MeSH heading, and 106,651 other entry points.
The MeSH thesaurus is used by the NLM for indexing articles from 4600 of
the world’s leading biomedical journals for the Medline repository and the
PubMed retrieval system.
The MeSH browser is a vocabulary lookup aid for a specific ontology. It is
designed to help quickly locate descriptors of possible interest and to show
the hierarchy in which descriptors of interest appear. The MeSH is a stand-
alone service that is not directly linked to any retrieval system.
The MeSH browser finds all matches to a query, and orders them alphabet-
ically by primary subject heading. One can ask for exact matches, matches to
all of the words in the query, or matches to any of the words in the query. The
entries and descriptors contain a great deal of information about the concept.
Searches can be restricted in a variety of ways.
The MeSH browser is a very useful browser for medical terminology, but
the MeSH thesaurus is very small compared to the UMLS, which was de-
scribed in more detail in subsection 5.1.1. The UMLS includes MeSH as well
as terminology from over 100 other sources. The UMLS itself can be licensed
from the NLM, and the distribution includes a simple browser. More sophis-
ticated browsers for the UMLS are available from a number of companies, ei-
ther commercially or for free on the web. Know-ME is freely available on the
web at (Know-Me 2004). It is not known how much of the UMLS is covered
by Know-ME. The Apelon DTS covers 8 of the over 100 source vocabularies
of the UMLS. It is commercially available, but freely available to government
employees. The SKIP Knowledge Browser from SemanTx Life Sciences (Jarg
2005) is available on the web atwww.semantxls.comand covers all of the
source vocabularies of the UMLS, subject to copyright restrictions.
The more sophisticated browsers described above allow one to use ontolo-
gies in a number of ways during information retrieval. The most important
feature of an ontology is its terminology. The terms of an ontology are called
thecontrolled vocabulary. They allow one to formulate a query using the key-
words that have been used in a corpus of documents. However, an ontology
consists of more than just a controlled vocabulary. It also organizes concepts
hierarchically and has many relationships between concepts. When one is
browsing an ontology, one can navigate from concept to concept in several
ways:
- One can use more general concepts, when more specific concepts do not
find the desired information. This is known as “broadening” the query.