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Table of Contents
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Introduction

A discussion paper from the Content Managers Advisory Group, entitled Discussion Paper - Allowance of Extensions to Modify Core Content was submitted to SNOMED International management to resolve issues surrounding the interpretation of the allowable changes national extensions may make to 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.  This response proposes a set of policy statements that explicitly state what can and cannot be done to content of the International release by national extensions.

While this response is principality focused on the needs of national extensions, the general policies discussed here will apply to all extensions, regardless if they are directly dependent on the International release or another edition.  

Background

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

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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:

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This document attempts to clarify the interpretation of Clause 4.1 and clarify the allowable changes that an extension may make in their module to address perceived issues with the International release.

Evaluation of proposed allowable actions for national extensions

The initial draft of this policy document was extensively discussed at the Editorial Advisory Group face-to-face meeting in London, 2017. The consensus at the conclusion of this discussion topic was:

• The interpretation by the EAG of section 2.1.4 was that of option A, i.e. "The RF2 distribution [of the International release] must not be modified... Overwriting data in the distribution files - such that an extension violates the append only model is not allowed...the full history [of] ...the international release must provided.
• Conversely, the full content of the original RF2 International release distribution must be extractable from any RF2 distribution of a extension edition (combination of International and extension content), without the need for any data transformation.
• The use cases for modifications to core international content were recognized as generally valid and as potentially causing classification changes to inferred views of international content in a extension release.
  
• All modifications to the International release must be appended to the RF2 files within a Module ID owned by the extension.  Changes to the SNOMED International Module are not allowed.
• Substantive errors in or improvement to content in the International release that are mitigated by content in an extension should be forwarded to SNOMED International in a timely fashion to improve the quality of the International release.

Summary of allowed actions by Extensions

In light of the above conclusions, append only changes may be made within an approved extension module, regardless of the impact on the inferred relationships of International release concepts, provided the following are adhered to:

• These allowances are ONLY applicable to RF2 releases, where a full history of content changes is distributed
• RF1 distributions that are derived from RF2 extensions that involve changes to inferences in the International release are in violation of Clause 4.1 of the affiliate license (source of the changes cannot be tracked, original International release cannot be extracted)
• Classification inferences in an edition that result in differences from the International release must not be represented as SNOMED International content.
• Due to the changes to International classifications caused by modifications by national extensions, a disclaimer notifying users of the differences between the extension release and the International release must accompany the national extension distribution files.

Actions not allowed by Extensions

In spite of the relative freedom implied by the above statements, there are some actions that can potentially be performed which violate the basic premise of achieving semantic interoperability or create structural or clinical quality issues, which must be avoided.  The following actions are not sanctioned, even if performed within the module of the extension

• Changes to the GB and US language refsets are not allowed.  Creation of a new dialect refset is the appropriate approach to specifying a particular preferred term.
• Creation of stated IS-A relationships that result in ancestors from mutiple top-level hierarchies. (e.g. Clinical finding and Procedure)
• Creation of concepts that are a duplication of existing International concepts.

Example disclaimer for extensions that modify core components

The __________ extension has added components that, following classification, change the inferred relationships of some International concepts. These changes result in differences between the International release and the _________ edition (International release plus extension content)  with regards to taxonomy and definitions of affected International release concepts. Implementers are cautioned to be aware of these differences when inferred relationships from this edition are used in analyses or when expanding intentional refsets.

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