A measurement function is the algorithm or calculation that combines two or more quality measure elements into a quality measure. It is the part of a measure that makes it reproducible. If the QME is the raw material, the measurement function is the recipe. The draft keeps the definition short: an algorithm or calculation performed on two or more quality measure elements. That brevity hides the decisions. A ratio needs a numerator and a denominator with the same population. A percentage needs a denominator that can reach the whole. A statistical function needs a distribution and a rule for outliers. The definition also implies that the function is fixed before the result is seen. A function chosen after the data arrives is a narrative, not a measure. The draft's Annex C names five measurement data types that a function can produce: count, time, ratio, percentage and statistical.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.