Centrality

= aspect of Network Theory

Definition

Mike Gotta:

"The concept of centrality, in simple terms, attempts to identify those actors in a network that appear to be highly connected. This is often referred to as “directed” centrality which looks at the number of direct incoming and out-going links. There are other ways of determining centrality. “Betweeness” centrality looks at actors that may not have a large number of direct links but they are in a position that lies between many other actors. “Closeness” centrality is somewhat self-defining - it applies to actors who are a short distance from other actors in the network." (http://mikeg.typepad.com/perceptions/2008/04/social-networks.html)

Description

"An understanding of social networks needs also to include accounts of centrality and of one node’s relationship to other nodes in a network. This is why Linton C. Freeman’s article on centrality in social networks is important (Freeman, 1979). Freeman explored how “graph centralization” was based on differences in point centralities. He also outlined three competing theories regarding the definition of centrality based on degree of a point, control and independence.

Degree of a point refers to the number of nodes connected to a given node. In simple terms, this means counting the number of friends you have in a social network. The more friends, you have, the more important you are.

Control refers to the extent to which nodes depend on one specific node to communicate with other nodes. For example, if hundreds of friends are connected to each other only when you serve as the bridge connecting them, then your centrality is high. You are the node that controls the communication flows.

And finally independence means that a node is closely related to all the nodes considered – so that it is minimally dependent on any single node and is not subject to control. This means you can reach the maximum number of people through the shortest number of links, without being dependent on a particular few nodes.

Because social networks are fundamentally social tools in which people are constantly monitoring and growing their social network, most social network media depict growth using the degree of point definition. However, control and independence can be more useful definitions. For example, a person who controls information flows is more important than one who may have more friends in the network. Centrality can also indicate which members are the most useful or well connected and therefore the best information resources." (http://www.boxesandarrows.com/view/social-networks)

Typology

Kumar, Novak and Tomkins also saw that network activity is of three types:

• “Singletons,” who have no connections and are least central

• The “giant component,” which is the largest group of nodes tightly connected to the central nodes and to each other

• The “middle region,” which represents isolated groups which interact amongst themselves but not with the rest of the network, forming isolated stars. These groups grow one user at a time. Over time they merge with the giant component."

(http://www.boxesandarrows.com/view/social-networks)

Mike Gotta:

"Centrality often leads to a discussion on the type of nodes (and roles they may play) within a social network:

• Connectors: are actors that have a large number of links directed towards them such that they function as a central hub.

• Brokers: are often actors that have a high degree of betweeness such that they are in a role of influence where they can acts as gatekeepers and filters for various relation flows.

• Boundary spanners: are actors that provide links between otherwise independent collections actors

• Peripheral actors: are nodes that are somewhat isolated and not linked to one another, there are often considered not active participants but may still play a key role (e.g., subject matter expert)"

(http://mikeg.typepad.com/perceptions/2008/04/social-networks.html)