A

Notes

• Two normal form expressions can be readily compared to determine whether they are logically equivalent 

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  or whether one 

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   is

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  t subsumed

   by the other.
• In theory, an 

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  can be transformed to its normal form by replacing each reference to a

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  with a nested

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   that represents the definition of that

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  .  However, this process often results in redundancy or duplication of meaning requiring removal of less specific

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   and mergers of 

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  t attribute groups

  . Therefore, use of

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  t description logic classifier

   is more effective way to normalize and compare 

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  .

Change Notice

• Prior to the July 2019 The January 2019 release of the 

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  t International Edition

   included , two normal form expressions could be compared to determine whether they were logically equivalent 

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   or whether one 

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  PreSpace false t expression

   was 

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  t subsumed

   by the other. The July 2019 release included enhancements to the 

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  t description logic

   used by 

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  t SNOMED CT

  . As a result of these enhancements, expression transformation is no longer a reliable option for 

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  t subsumption testing

  . Instead, postcoordinated expressions should be classified using a 

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  t description logic classifier

   to determine subsumption.

Related Links

• Gloss
  t Canonical form

• Terminology Services Guide
  • 12.4 Transforming Expressions to Normal Forms
• Description Logic Enhancements
  
  • SNOMED CT Description Logic Profile Specification
  • DOCOWL