Introduction

A discussion paper from the Content Managers Advisory Group, entitled Discussion Paper - Allowance of Extensions to Modify Core Content has been submitted to SNOMED International management to resolve issues surrounding the interpretation of what national extensions are allowed to modify in the International release of SNOMED CT.  The discussion paper outlines a number of proposed clarifications to the interpretation of what is allowed and details the requirements for those changes.

Background

The primary concern expressed is the interpretation of Clause 4.1 of the SNOMED CT® AFFILIATE LICENSE AGREEMENT, which states: 

“Subject to clause 2.1.4, the Licensee may not modify any part of the SNOMED CT Core distributed as part of the International Release or as part of a Member’s National Release.”

Clause 2.1.4 states:

 “(The Licensor grants the Licensee, license to) modify the manner of formatting of the copy of the SNOMED CT Core distributed to the Licensee as part of the International Release or as part of a Member’s National Release”

 According to the discussion paper, there are two primary issues associated with this clause:

 "This restriction on modifications has been interpreted generally in two ways. 

1. The RF2 distribution must not be modified, beyond appending additional rows. Overwriting data in the distribution files - such that an extension violates the append only model is not allowed (editor's addition). Additionally, the full history provided by the international release must provided, i.e. extensions may not omit anything;

2. Core components must not be modified.
   Overriding the international content through either:

• the addition of new versions for core components within extensions; or

• the addition of relationships to core concepts and changing their DL definition.

Restrictions covered by B have been shown to be impractical and prohibit proper quality terminology authoring."

Proposed actions allowed for National Extensions

The discussion paper proposes seven actions that national extensions may want to perform in the course of their content development and maintenance:

1. Create new concepts.

2. Fully define concepts they create.

3. Classify terminology extensions.

4. State additional IS A relationships against core (international) concepts.

5. Retire (redundant) IS A relationships (not necessarily stated).

6. Add additional defining (non-IS A) relationships to primitive core concepts.

7. Retire content considered "inappropriate" - concepts, descriptions or relationships

The paper then goes into substantial detail justifying the need to perform each of these actions.  A request was forwarded to SNOMED International management for review and clarification of the interpretation of clause 4.1 and acceptance of the allowable proposed actions available to national extension managers.

This document attempts to clarify the feasibility of each of the proposed actions and provides justification for the acceptance or rejection of each, in order to make the interpretation of Clause 4.1 more clear.

Evaluation of proposed allowable actions for national extensions

Create new concepts

The current interpretation of Clause 4.1 with regards to creation of new concepts is that the new concepts would necessarily be leaf nodes to the existing International release.  This type of content, by its very nature would have no impact on the international content, as it would be distal in the hierarchy to all international content.  National extensions have found that, due to gaps in the current International release, it is often necessary to insert intermediate (i.e. non-leaf) concepts into the terminology to meet user requests as well as to provide additional navigational support.  These intermediate concepts may be either primitive or fully-defined.

The creation of intermediate concepts necessarily results in changes to the relationships in the core terminology.  These changes can be of almost any type including: assignment of new parent(s), retirement of inferred or stated parents, inheritance of new/additional defining relationships, removal of inferred relationships by the classifier, etc. 

Proposed policy

SNOMED International recognizes that SNOMED CT is not always comprehensive in its terminological coverage in particular domains and that there exist "gaps" in the hierarchies, where concept coverage is not adequate for extensions.  It is proposed that intermediate concepts may be added to extensions under the following guidelines:

1. The addition of the concept does not inactivate existing inferred IS A relationships except to insert a new proximal ancestor (the intermediate concept); i.e. does not change the International release transitive closure table (except to add the additional row representing the new intermediate ancestor relationship).  This necessitates the retention of transitive IS A relationships from the core.
2. No non-IS A inferred relationships of core content are inactivated
3. Additional inferred defining relationships to not create crossovers
4. Additional inferred IS A relationships of subtypes do not represent a change in meaning. 

Fully define concepts they create

Full definition of intermediate concepts will in many cases result in the reassignment of core relationship due to inferences assigned from the classifier (see next section).

Proposed policy

Classify terminology extensions

The ability to classify extensions is an essential part of leveraging the description logics represented in the International Release for the proper maintenance of extension content.  However, 

Proposed policy

State additional IS A relationships against core (international) concepts

Proposed policy

Retire (redundant) IS A relationships (not necessarily stated)

Proposed policy

Add additional defining (non-IS A) relationships to primitive core concepts

Proposed policy

Retire content considered "inappropriate" - concepts, descriptions or relationships

Proposed policy