As a quantitative measure of relationships, including those of classic rock artists above, social network analysis employs a unique vocabulary. Here is a brief rundown of some of the key terms employed:
Node: The subject being measured, whether a person or an organization. For a terrorist network, nodes might be the various members of the terrorist group.
Edges (or ties): Links between two nodes, or people, in the network.
Network: The term used to describe the relationship of the connected nodes. This could be a terrorist group, such as Al Qaeda, or a group of interconnected friends.
Graphs: A visual depiction of the network, represented by connecting lines between the nodes.
Bridge: A node that forms a critical tie between two different cluster of nodes, or groups. This could be, for example, a key player in a terrorist organization.
Density: The number of ties in a network measured as a proportion of the total number of possible connections or ties.
Cohesion: The degree to which nodes in a network are connected, which can be measured by distance between the nodes (how many steps, or “degrees of separation” there are between any two nodes).
Reachability: The number of pathways between two nodes in the network, which measures the ability to make connections.
Centrality: The importance of a node in the network, which helps assess the node’s ability to make new connections, and thus can also be used to assess the node’s importance within the network. Centrality is measured by: degree, the number of direct connections a node has to other nodes; closeness, the shortest path between two nodes, and betweenness, how often a node lies on the shortest path between two other nodes on the network. (With regard to a terrorist network, Valdis Krebs writes that 9/11 ringleader Mohamed Atta “scores the highest on all network centrality metrics.” Meaning, Atta had the most direct connections to other members of the 9/11 network, the greatest ability to connect with other members, and was in the best position to facilitate or broker connections.)