Table 1.

Description of injection drug network characteristics

Network metric	Definition	Network feature captured
Degree	In-degree + out-degree	Number of injecting partners
Eccentricity	Largest geodesic distance – the largest number of steps in the shortest possible walk from one PWID to every other PWID in the network	How far each PWID is from the furthest other in the network
Clustering coefficient	The ratio of actual edges between a PWID node and its neighbours to the total number of potential edges	The number of injecting partners relative to the total possible number of injecting partners
Closeness centrality	Inverse of the sum of distances to all other PWID	A measure of the centrality of a PWID within the injection drug network
Betweenness centrality	The number of shortest paths from all nodes to all other PWID that pass through a particular PWID node	The number of times a PWID acts as a bridge along the shortest path between two other PWID
Eigenvector centrality	How connected a PWID is those parts of the injecting network with the greatest connectivity	The extent to which ‘big fish’ connect with other ‘big fish’