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The use of description logic as the formal foundation of SNOMED CT allows the semantics of clinical concepts to be represented unambiguously. Description logic also enables logical deduction in which additional information can be inferred from the explicit statements in the terminology. Classification is the process in which the formally stated definitions of each concept are used to compute the subsumption hierarchies and defining properties of each concept.
With every each new release of SNOMED CT, two types of relationship files are distributed: the Stated Relationship file, containing the defining relationships stated by the terminology authors; and the Relationship file, containing the stated concept definitions are represented in the OWL Expression reference set, while the Relationship file, contains the full set of relationships that can be inferred using a classifier (excluding non-redundant
| Concept |
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| t | 116680003 |is a| |
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relationships).
It is important that extension producers are aware of the difference between these two files, and the fact that most Most consumers of the
terminology SNOMED CT will use the Relationship file (containing the inferred relationships). The
Stated Relationship file is OWL reference sets are primarily used by extension producers, but may also be used by terminology consumers that have access to a description logic classifier.
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OWL Expression Reference Set
| Include Page |
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| DOCRELFMT:OWL Expression Reference Set file |
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| DOCRELFMT:OWL Expression Reference Set file | DOCTSG:9.1 Stated Relationships File | DOCTSG:9.1 Stated Relationships File |
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Inferred Relationships
Inferred relationships are derived from the set of stated defining relationshipsof
in the OWL Expression Reference Set, by applying a consistent set of logical rules to the definition which take account of the definitions of related
concepts.
Several semantically equivalent views may be inferred from the same set of stated relationships. However, the SNOMED CT Relationship file is distributed using an inferred view known as the Distribution Normal Form (DNF)
| Gloss |
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| t | necessary normal form (NNF) |
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| GlosTerm | necessary normal form |
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. This standard distribution view, includes all non-redundant
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| t | 116680003 |is a| |
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relationships (between each concept and its proximal supertype), and the inferred concept definition of each concept (including all non-redundant defining relationships).
For more information, please refer to the Inferred definition views section in the Terminology Services Guide. Please note that in the standard distribution view (DNF):
- Redundant
| Concept |
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| ShowParts | term |
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| t | 116680003 |Is a| |
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relationships are removed - Defining attribute relationships are inherited from the concept's supertype parent, unless the attribute is also stated with a more specific value
In the necessary normal view (NNF) appropriate subtype and attribute relationships are created to represent non-redundant axioms that are necessarily true capable of being represented using the relationship file format.
| Note |
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Some more advanced axioms (e.g. | Gloss |
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| t | general class inclusions |
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) can be represented in the OWL Expression Reference Set but cannot be represented in the Relationship File. |
Example
This example is included to illustrate what happens during the classification process.
Consider the stated relationships shown below for the concepts
| Concept |
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| ShowParts | term |
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| t | 80146002 |Appendectomy| |
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and
| Concept |
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| ShowParts | term |
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| t | 174036004 |Emergency appendectomy| |
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. The concept definitions are represented in accordance with
SNOMED CT Diagramming Guidelines.
| Caption label |
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| CapId | example-stated-relationships |
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| CapType | Table |
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Example stated relationships |
This diagram shows the stated definition of the concept | Concept |
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| ShowParts | term |
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| t | 80146002 |Appendectomy| |
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. An | Concept |
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| ShowParts | term |
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| t | 80146002 |Appendectomy| |
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is a procedure in which the appendix structure is excised. | Stated definition of |Appendectomy|  Image Modified
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This diagram shows the stated definition of the concept | Concept |
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| ShowParts | term |
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| t | 174036004 |Emergency appendectomy| |
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. An | Concept |
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| ShowParts | term |
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| t | 174036004 |Emergency appendectomy| |
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is a procedure with a priority of emergency, in which the appendix structure is excised. | Stated definition of |Emergency appendectomy|  Image Modified
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| This diagram shows a hierarchical view of the stated subtype relationships from the concepts above. | Stated subtype relationships  Image Modified
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| Caption label |
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| CapId | comparison-of-stated-definitions |
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| CapType | Table |
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Comparison of stated definitions |
This diagram shows a comparison of the stated definitions for | Concept |
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| ShowParts | term |
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| t | 80146002 |Appendectomy| |
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and | Concept |
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| ShowParts | term |
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| t | 174036004 |Emergency appendectomy| |
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from above. The two definitions are identical, except for the additional defining relationship on | Concept |
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| ShowParts | term |
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| t | 174036004 |Emergency appendectomy| |
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, which states that the priority is emergency.This means that a classifier can infer that | Concept |
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| ShowParts | term |
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| t | 174036004 |Emergency appendectomy| |
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is a logical subtype of | Concept |
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| ShowParts | term |
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| t | 80146002 |Appendectomy| |
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. |  Image Modified
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| Caption label |
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| CapId | deriving-the-inferred-relationships |
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| CapType | Table |
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Deriving the inferred relationships |
This diagram shows that once the relationship | Concept |
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| ShowParts | term |
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| t | 174036004 |Emergency appendectomy| |
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| Concept |
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| ShowParts | term |
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| t | 116680003 |is a| |
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| Concept |
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| ShowParts | term |
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| t | 80146002 |Appendectomy| |
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|
is inferred, the stated relationship | Concept |
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| ShowParts | term |
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| t | 174036004 |Emergency appendectomy| |
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|
| Concept |
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| ShowParts | term |
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| t | 116680003 |is a| |
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|
| Concept |
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| ShowParts | term |
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| t | 71388002 |Procedure| |
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becomes redundant. |
 Image Modified
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Combining Modules for Classification
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The process of combining an extension with the content of the modules on which it depends is illustrated in below.
Image Removedin
| Caption reference |
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| CapRefId | creating-inferred-relationships-of-extensions |
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| CapRefType | Figure |
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below.Image Added
| Caption label |
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| CapId | creating-inferred-relationships-of-extensions |
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| CapType | Figure |
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Creating inferred relationships for an extension |
It is important that all inferred relationships present in the International Edition are also present in the combined inferred view. This means that the Relationship file in an extension's edition should be a superset of the Relationship file in the International Edition. In particular:
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