Quality measures are often derived from clinical guidelines and are designed to determine whether the appropriate care has been provided given a set of clinical criteria and an evidence base. Note that the Measure itself does not typically contain any logic; rather a Library resource is referenced that contains the logic required by the measure, and the various expression elements, such as population criteria, reference named expressions within that library (or libraries). In addition, if the Measure references multiple libraries, then any expression references within the resource must be qualified with the name of the library that contains the referenced expression.
The Measure resource describes a specific quality measure, or population analytic, providing the structure of the measure in terms of the calculation elements (the populations involved). The Group resource is also capable of describing a population, however, the complexity involved in specifying the criteria in the general case requires the use of a high-level query language such as Clinical Quality Language (CQL). As such, the Measure resource defines only the top-level populations and references expressions for the actual criteria. These expressions are typically provided using a Library resource containing CQL or ELM expressions. In addition, the individual members of a population may be cases such as encounters or procedures and, in these cases, the Group resource would be unable to represent the population characteristics accurately.
A Measure is also similar to an Observation resource, with the exception that the Measure is purely definitional, it contains no actual measurements, only a description of how to calculate a particular measurement or set of measurements. A Measure is also similar to a clinical document, but as with the relationship to Observation, a Document is specific to a particular subject.