Stated and Inferred Concept Definitions
This section briefly outlines two distinct types of views of SNOMED CT concept definitions. More detailed illustrations of some of the points on this page are provided in Stated and Inferred Definitions - Examples.
Stated View of Concept Definitions
SNOMED CT concepts are defined by assertions made by SNOMED CT authors. The concept definitions asserted by SNOMED CT authors are known as the stated view.
The stated view is a representation of concept definitions consisting only of assertions made or revised by SNOMED CT authors.
Notes
In contrast to the inferred view, the stated view does not include inferences generated by applying a description logic classifier.
The stated view is represented by axioms, that conform to the OWL functional syntax. These axioms are distributed in the OWL axiom reference set file
Description Logic Classification
A description logic classifier can apply logical rules to the stated view to create inferences. The end result of this process is an inferred view of concept definitions.
Inferred Views of Concept Definitions
The inferred view is a representation of concept definitions that is logically derived by applying a description logic classifier to the stated view.
Notes
Different inferred views can be derived from the same stated view by applying different rules that selectively exclude some types of assertions.
Different inferred views may be semantically equivalent to one another provided that assertions are only excluded if they are redundant (i.e. can be inferred from assertions that are included). However, in some cases, an inferred view may not completely represent the concept definition but may serve a specific purpose.
Choosing the View to Use
The decision on whether you need access to the stated view and/or inferred view depends on your answers to the following questions.
If you answer "yes" to any of the following questions you need access to the stated view
Do you already use or plan to use a description logic classifier with SNOMED CT?
Are you responsible for or planning to create a SNOMED CT extension in which you will add new clinical concepts?
Do you need to query or analyze postcoordinated expressions in records?
Are you interested in experimenting with the application of description logic to SNOMED CT (e.g for educational reasons)?
If you answered yes to one these questions above, you should still check the next set of questions to consider if you would also benefit from accessing the inferred view. On the other hand, if you apply a classifier to the stated view you could generate your own inferred view.
If you answer "yes" to any of the following questions you need access to an inferred view
Do you need access to a simple relational table representation of the defining relationships between concepts?
Do you need to be able to display or navigate the subtype hierarchy using only the links between each concept and its proximal subtypes and proximal supertypes?
Do you need to generate a transitive closure view of the subtype hierarchy to assist with rapid subsumption testing?
Several different inferred views can be derived from a single stated view. The inferred view distributed as part of the SNOMED CT release conforms to the necessary normal form.
Note: The necessary normal form (NNF) is similar to the distribution normal form (DNF) that was released prior to the updates in 2018. However, the rules for Generating Necessary Normal Form take account of the need to omit more advanced features that cannot be represented as relationships.
If you answer "yes" to any of the following questions you do not need direct assess to either view
Does an application you are responsible for use software or services provided by another organization to access SNOMED CT?
Are you an end-user of a software application that provides access to SNOMED CT?
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