Original Source
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What are the differences between a vocabulary, a taxonomy, a thesaurus, an ontology, and a meta-model?
Wednesday, January 15 2003 @ 09:12 PM PST
Contributed by: jernst
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This excellent overview was contributed by Woody Pidcock of the Boeing company.
Many organizations and companies are struggling with these terms and the ideas
behind them; this set of definitions will help to clarify.
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I will answer this question one step at a time. To keep this answer focused on
the question, I will use other concepts that I will not define here. If this
generates additional questions, feel free to comment on this post.
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A controlled vocabulary is a list of terms that have been
enumerated explicitly. This list is controlled by and is available from a
controlled vocabulary registration authority. All terms in a controlled
vocabulary should have an unambiguous, non-redundant definition. This is a
design goal that may not be true in practice. It depends on how strict the
controlled vocabulary registration authority is regarding registration of terms
into a controlled vocabulary. At a minimum, the following two rules should be
enforced:
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- If the same term is commonly used to mean different concepts in
different contexts, then its name is explicitly qualified to resolve
this ambiguity.
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- If multiple terms are used to mean the same thing, one of the
terms is identified as the preferred term in the controlled vocabulary
and the other terms are listed as synonyms or aliases.
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A taxonomy is a collection of controlled vocabulary terms
organized into a hierarchical structure. Each term in a taxonomy is in one or
more parent-child relationships to other terms in the taxonomy. There may be
different types of parent-child relationships in a taxonomy (e.g., whole-part,
genus-species, type-instance), but good practice limits all parent-child
relationships to a single parent to be of the same type. Some taxonomies
allow poly-hierarchy, which means that a term can have multiple parents. This
means that if a term appears in multiple places in a taxonomy, then it is the
same term. Specifically, if a term has children in one place in a taxonomy,
then it has the same children in every other place where it appears.
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A thesaurus is a networked collection of controlled vocabulary
terms. This means that a thesaurus uses associative relationships in addition
to parent-child relationships. The expressiveness of the associative
relationships in a thesaurus vary and can be as simple as “related to term” as
in term A is related to term B.
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People use the word ontology to mean different things, e.g.
glossaries & data dictionaries, thesauri & taxonomies, schemas & data models,
and formal ontologies & inference. A formal ontology is a controlled vocabulary
expressed in an ontology representation language. This language has a grammar
for using vocabulary terms to express something meaningful within a specified
domain of interest. The grammar contains formal constraints (e.g., specifies
what it means to be a well-formed statement, assertion, query, etc.) on how
terms in the ontology’s controlled vocabulary can be used together.
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People make commitments to use a specific controlled vocabulary or
ontology for a domain of interest. Enforcement of an ontology's grammar may be
rigorous or lax. Frequently, the grammar for a "light-weight" ontology is not
completely specified, i.e., it has implicit rules that are not explicitly
documented.
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A meta-model is an explicit model of the constructs and rules
needed to build specific models within a domain of interest. A valid
meta-model is an ontology, but not all ontologies are modeled explicitly as
meta-models. A meta-model can be viewed from three different perspectives:
- as a set of building blocks and rules used to build models
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- as a model of a domain of interest, and
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- as an instance of another model.
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When comparing meta-models to ontologies, we are talking about
meta-models as models (perspective 2).
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Note: Meta-modeling as a domain of interest can have its own ontology. For
example, the CDIF Family
of Standards, which contains the CDIF Meta-meta-model along with rules for
modeling and extensibility and transfer format, is such an ontology. When
modelers use a modeling tool to construct models, they are making a commitment
to use the ontology implemented in the modeling tool. This model making
ontology is usually called a meta-model, with “model making” as its domain of
interest.
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Bottom line: Taxonomies and Thesauri may relate terms in a controlled
vocabulary via parent-child and associative relationships, but do not contain
explicit grammar rules to constrain how to use controlled vocabulary terms to
express (model) something meaningful within a domain of interest. A meta-model
is an ontology used by modelers. People make commitments to use a specific
controlled vocabulary or ontology for a domain of interest.
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