This case-sensitive code system http://hl7.org/fhir/us/cqfmeasures/CodeSystem/aggregate-method defines the following codes:

Code	Display	Definition
sum	Sum	The measure score is determined by adding together the observations derived from the measure population.
average	Average	The measure score is determined by taking the average of the observations derived from the measure population.
median	Median	The measure score is determined by taking the median of the observations derived from the measure population.
minimum	Minimum	The measure score is determined by taking the minimum of the observations derived from the measure population.
maximum	Maximum	The measure score is determined by taking the maximum of the observations derived from the measure population.
count	Count	The measure score is determined as the number of observations derived from the measure population.