EBMonFHIR, CPGonFHIR, and related projects desire to use Group Resource to define the sample that evidence is derived from, the population that evidence is interpreted for, the population that a recommendation is intended for, and the population that clinical decision support artifacts are intended for. In actual implementations of clinical decision support the Group Definition is applied through expressions in CQL and not in the Group Resource. With a desire to convey Group Definitions in Group Resource and support interoperability with transition to CQL we have a desire to express some Group characteristics with an Expression datatype rather than CodeableConcept, Quantity, Range, Boolean, or Reference.

An example of a Group Resource with 5 inclusion criteria and 15 exclusion criteria that would use valueExpression for some of these criteria (those with Measurement values as the type of characteristic) can be found at https://fevir.net/resources/Group/18125