Katz centrality
Encyclopedia
In Social Network Analysis (SNA) there are various measures of centrality
which determine the relative importance of an actor (or node) within the network. Katz centrality was introduced by Leo Katz in 1953 and is used to measure the degree of influence of an actor in a social network. Unlike typical centrality measures which consider only the shortest path (the geodesic
) between a pair of actors, Katz centrality measures influence by taking into account the total number of walks between a pair of actors.
.
For example, in the figure on the right, assuming that John’s centrality is being measured and that α
. The weight assigned to each link that connects John with his immediate neighbors Jane and Bob will be 
. Jose connects to John indirectly through Bob, the weight assigned to this connection (composed of two links) will be 
. Similarly, weight assigned to the connection between Agneta and John will be 
.
of a network under consideration. Elements
of A are variables that take a value 1 if a node i is connected to node j and 0 otherwise. The powers of A indicate the presence (or absence) of links between two nodes through intermediaries. For instance, in matrix 
, if element 
, it indicates that node 2 and node 12 are connected through some first and second degree neighbors of node 2. If 
denotes Katz centrality of a node i, then mathematically:
The value of the attenuation factor α has to be chosen such that it is smaller than the reciprocal of the absolute value of the largest eigenvalue of the adjacency matrix A. In this case the following expression can be used to calculate Katz centrality:
Here I is the identity matrix,
is an identity vector of size n (n is the number of nodes) consisting of ones. 
denotes the transposed matrix of A and 
denotes matrix inversion of the term 
.
Centrality
Within graph theory and network analysis, there are various measures of the centrality of a vertex within a graph that determine the relative importance of a vertex within the graph...
which determine the relative importance of an actor (or node) within the network. Katz centrality was introduced by Leo Katz in 1953 and is used to measure the degree of influence of an actor in a social network. Unlike typical centrality measures which consider only the shortest path (the geodesic
Geodesic
In mathematics, a geodesic is a generalization of the notion of a "straight line" to "curved spaces". In the presence of a Riemannian metric, geodesics are defined to be the shortest path between points in the space...
) between a pair of actors, Katz centrality measures influence by taking into account the total number of walks between a pair of actors.
Measuring Katz centrality
Katz centrality computes the relative influence of a node within a network by measuring the number of the immediate neighbors (first degree nodes) and also all other nodes in the network that connect to the node under consideration through these immediate neighbors. Connections made with distant neighbors are, however, penalized by an attenuation factor α. Each path or connection between a pair of nodes is assigned a weight determined by α and the distance between nodes as α.For example, in the figure on the right, assuming that John’s centrality is being measured and that α
. The weight assigned to each link that connects John with his immediate neighbors Jane and Bob will be . Jose connects to John indirectly through Bob, the weight assigned to this connection (composed of two links) will be . Similarly, weight assigned to the connection between Agneta and John will be .Mathematical formulation
Let A be the adjacency matrixAdjacency matrix
In mathematics and computer science, an adjacency matrix is a means of representing which vertices of a graph are adjacent to which other vertices...
of a network under consideration. Elements
of A are variables that take a value 1 if a node i is connected to node j and 0 otherwise. The powers of A indicate the presence (or absence) of links between two nodes through intermediaries. For instance, in matrix , if element , it indicates that node 2 and node 12 are connected through some first and second degree neighbors of node 2. If denotes Katz centrality of a node i, then mathematically:The value of the attenuation factor α has to be chosen such that it is smaller than the reciprocal of the absolute value of the largest eigenvalue of the adjacency matrix A. In this case the following expression can be used to calculate Katz centrality:
Here I is the identity matrix,
is an identity vector of size n (n is the number of nodes) consisting of ones. denotes the transposed matrix of A and denotes matrix inversion of the term .