Recommended Update: Add general set operators to more accurately and consistently organize logic.

The “union” set operator provides clarity in the specification of clinical quality measures. Currently, measures can incorporate subset operators like “first” or “most recent” with logical conditions underneath them. The logical conditions below a subset operator can be either an AND or an OR. However, there is no distinction between the use of ANDs or ORs below a subset operator. For instance, there is no difference between the first and second logical block below:
First logical block:

AND: COUNT > 3
    OR: Encounter X
    OR: Encounter Y

Second logical block:

AND: COUNT > 3
    AND: Encounter X
    AND: Encounter Y

Furthermore, the use of the logical constructs in this case is not appropriate to the definition. Logical operators like OR, should produce a Boolean value. However, subset operators are intended to operate against a set of elements. Therefore, it is more consistent to use set operators like union and intersection below a subset operator. That would produce the following logic:

AND: COUNT > 3
    UNION: 
        Encounter X
        Encounter Y