Recommended Update: Introduce a variable assignments function to the QDM that identifies sets of data elements for simplified reference and reduced repetition in a measure.

Variable assignments are a construct that allows for the definition of sets of data elements and the assignment of those sets to a named variable that can be referenced within the measure logic. The value of variable assignment is that it allows complicated logical statements building a set of elements to be extracted from the measure logic into a named element. This allows redundant specifications of the logic to be eliminated allowing the measure logic to simply reference the named variable multiple times. Additionally, named variables allow similar data elements to be joined together to simplify permutations of logic covering multiple combinations of logic. The joining of similar concepts can currently be done using grouping value sets, however, using value sets to simplify measure logic leads to measure specific value sets that do not always align well with realistic clinical concepts. Variable assignments allow grouping together concepts within the context of the measure logic without having to introduce new value sets. Consider the following:

AND: 
    OR: Medication, Order A during Encounter, Performed X 
    OR: Medication, Order B during Encounter, Performed X 
    OR: Medication, Order A during Encounter, Performed Y 
    OR: Medication, Order B during Encounter, Performed Y 

In this case we have two medications and two types of encounter. However since the encounters and medications are not grouped, we must have logic that compares each combination of medication and encounter. There could be additional logic around the medication or encounter that could complicate the permutations of logic even further. However, with variable assignments, the medications and encounters can be grouped simplifying the branching of the logic along various OR paths for each permutation.

TargetMedications =
    OR: Medication, Order A 
    OR: Medication, Order B 

TargetEncounters =
    OR: Encounter, Performed X
    OR: Encounter, Performed Y

AND: $TargetMedications during $TargetEncounters

The construction of the target sets could contain additional logic including temporal references or subset operators. For instance each of the target encounters could be restricted to the measurement period. The use of variable assignments allows logic to be extracted from the measure definition and centralized.
In addition to the use of set operators with subset operators, set operators are also used to define the sets used as part of variable assignments (see QDM-9).