# normal form

## normal form

A SNOMED CT expression in which none of the referenced concepts are fully defined and where there is no redundancy or duplication of meaning.

#### Notes

* In theory, an expression can be transformed to its _normal form_ by replacing each reference to a fully defined concept with a nested expression that represents the definition of that concept. However, this process often results in redundancy or duplication of meaning requiring removal of less specific attribute values and mergers of attribute groups. Therefore, use of description logic classifier is more effective way to normalize and compare expressions.

#### Change Notice

* Prior to the July 2019 International Edition, two _normal form_ expressions could be compared to determine whether they were logically equivalent equivalence or whether one expression was subsumed by the other. The July 2019 release included enhancements to the description logic used by SNOMED CT. As a result of these enhancements, expression transformation is no longer a reliable option for subsumption testing. Instead, postcoordinated expressions should be classified using a description logic classifier to determine subsumption.



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