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Monday, January 28, 2013

The Model is the Standard

In HL7 Version 3, with one significant exception, the information model derived from the RIM is the normative expression of the standard.  The significant exception is of course CDA, which has the normative requirement that a CDA instance must validate according to the XML Schema produced from the model (once all extension elements and attributes are removed).

In HeD and HQMF, we want the overlapping concepts (the data to be used, and the operations that can be performed over them) to be drawn from the same model.  The model thus provides consistent semantics, and we can trace from clinical guidance to both the measures of implementation and the clinical decision support that implements it.

We had a long discussion today between myself, and members of the HeD development team to figure out how to move forward.  We have, as a result a proposal that stems from the fact that the V3 schemas are not normative.  Here is a quick summary of the proposal from my perspective:

  1. The current HeD guide is treated as an implementation guide of a to be developed standard model, and is altered somewhat, but not substantially so, to be published by HL7 as an informative document or DSTU.  The alterations to be made stem from current ballot comments, and include harmonization with the HQMF header structure, metadata, and outline.  This does not include changes to its use of VMR, representations of logic or expressions.
  2. A new HeD standard model will be produced subsequently that is compatible with this implementation guide.  It will use the same model elements as HQMF where there is overlap.  The VMR/QDM data access layers and the logic/expression evaluation will be separate components.
  3. In that standard, the necessary logic and expression evaluation capabilities will be described functionally based on the capabilities of the existing HeD expression language.  These will be aligned with the simple expression language (based on simple math) that the SD and CDS workgroups agreed to in our joint session as being an implementation feature of the QDM-based HQMF guides.  Thus, you could map from something like simple math to the HeD XML expression representation if you wanted.  The capabilities would be the same (or very nearly so).
  4. Adjuncts can be produced which transform from the HL7 V3 schemas generated automatically by our existing tooling, into the language specified in the HeD implementation guide.
  5. We can also look at alternative ITS representations that allow the separation of models (e.g., VMR/QDM) from schema representations, to better support implementation flexibility.
This proposal keeps existing HeD pilots on their existing timelines as if my major ballot comments had not been made, aligns the current guide with the HQMF header and outline, allows HQMF to move forward on roughly the expected timelines we expect, and still harmonizes the efforts of both groups.

This was an exercise in making things possible, not in making them perfect, but I think we have a perfectly possible solution.

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