**4. Representation of objects of quality indicators**

In this subsection, we define a graph that represents a target of quantification based on the medical service ontology defined in the previous subsection. We call such a graph an "objective graph". An objective graph is defined as a finite and labelled directed graph with a root node. A node in an objective graph is labelled by an instance of a concept or a value of an attribute of a concept in MSO, while an edge in an objective graph is labelled by an instance of a property in MSO.
