The aggregateMethod value set was defined in FHIR rather than using the HL7 V3 codes used in HQMF. This was a conscious decision to provide a clean slate, but the IG should provide a concept mapping:

V3 ValueSet: https://terminology.hl7.org/ValueSet-v3-ObservationMethodAggregate.html

FHIR ValueSet: http://hl7.org/fhir/us/cqfmeasures/ValueSet-aggregate-method.html

V3 > FHIR

SUM > sum

AVERAGE > average

MEDIAN > median

MIN > minimum

MAX > maximum

COUNT > count

Note that this is also related to the EBM-on-FHIR statistic type codesystem/valueset discussion, currently ongoing. Proposed resolution is to work with the EBM-on-FHIR group to define a new code system for statistic type, and then use a value set of those codes here. The justification for not using the HL7 V3 ObservationMethod code system is that what we are trying to represent here (types of statistics) is not the same thing as observation methods defined in that code system, most of which are actual clinical observation methods.