Glossary of Network Terms

Betweenness centrality	How central a node or a physician is within his network, obtained by considering shortest paths from each node to every other node in the network (Figure 2).
Bipartite network	“Bipartite” refers to a network where the nodes can be partitioned into two sets, here physicians and patients, such that all ties link nodes from one set to the other and there are no ties within a set. We convert this to a unipartite of linked physicians.
Clustering coefficient	The proportion of network neighbors of a node that are directly connected to one another, here a proportion of physician’s colleagues who share patients with one another.
Connection, edge, or tie	A tie connects two nodes, in this case linking two physicians in the network who share patients as identified in Medicare claims data. Connections likely correspond to information sharing relationships between physicians.
Degree, adjusted degree	The number of ties a given node has, here the number of doctors a physician is connected to through patient sharing. Because patient volume influences the number of connections, we obtain adjusted degree by dividing degree by the total number of Medicare patients the physician shares with all other doctors.
Homophily	The tendency of individuals with similar characteristics to associate with one another.
Node or actor	An individual actor or agent in the network, in this case a physician.
Relative betweenness centrality	The mean betweenness centrality of one physician type (e.g., PCP) relative to all other physicians in the network.
Shared patients	The total number of shared patients across all ties for an individual physician.
Social network	A set of actors, in this case physicians, and a set of relationships linking the actors together. Social networks can be used to study the structure of a social organization and how this structure influences the behavior of individual actors.