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Ordered reference sets can  can be used to specify and display a customized navigation hierarchy. Alternative hierarchical  Alternative hierarchical representations of SNOMED CT can support data entry by by satisfying the requirements of a specific use case, and addressing some of the challenges related to the poly-hierarchical and unordered structure of the subtype hierarchy of SNOMED CT.of displaying an unordered polyhierarchy (as defined by SNOMED CT's subtype structure).

The Ordered reference sets can be used to specify and display a customized navigation hierarchy. The figure below shows the way a navigation hierarchy is represented. The example reference set contains a set of concepts used to describe x-ray examinations of the upper and lower limbs. The resulting navigation hierarchy might usefully be extended to include other x-ray procedures but has been kept small for the purposes of the example. 

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• Constraining

Navigation Hierarchy example. SNOMED CT concepts can be organized in an alternative hierarchical view using an Ordered reference set.

Aside from providing an alternative hierarchical order, different features can be added to increase the usability of the reference set:

• Usability constraints can be placed on the number of levels in the hierarchy and/or the number of options displayed concepts at each level in the navigational hierarchy:.
  • Using many levels, each with a relatively small number of concepts, allows
  • If there are relatively few options and many layers, the most common options can to be brought to displayed with a higher level.If there are long lists of options, these may be subdivided with less frequent options moved to lower levelspriority.
  • Using fewer levels, each with a relatively large number of concepts can reduce the number of levels that needs to be navigated to find an appropriate concept.
  • Options that are never (or rarely or never used by a particular user community ) used can be excluded from a navigational customized navigation hierarchy to limit the range of choices . According to requirements, these options may remain accessible by switching to a subtype view.
• Options at each hierarchical level can be ordered to meet the expectations of users and/or to facilitate rapid access to commonly used options.
The available options at a particular level can be kept stable across releases without affecting the accuracy of the subtype hierarchy.
• • available.

• Ordering each concept at the same hierarchical level, to match user preferences or to facilitate faster access to more frequently used options.
• Ensuring that the navigation hierarchy is adapted to meet the requirements of a specific use case, without affecting the correctness of the subtype hierarchy (and associated logical inferences).