# power_centrality

##### Find Bonacich Power Centrality Scores of Network Positions

`power_centrality`

takes a graph (`dat`

) and returns the Boncich power
centralities of positions (selected by `nodes`

). The decay rate for
power contributions is specified by `exponent`

(1 by default).

- Keywords
- graphs

##### Usage

```
power_centrality(graph, nodes = V(graph), loops = FALSE, exponent = 1,
rescale = FALSE, tol = 1e-07, sparse = TRUE)
```

##### Arguments

- graph
- the input graph.
- nodes
- vertex sequence indicating which vertices are to be included in the calculation. By default, all vertices are included.
- loops
- boolean indicating whether or not the diagonal should be
treated as valid data. Set this true if and only if the data can contain
loops.
`loops`

is`FALSE`

by default. - exponent
- exponent (decay rate) for the Bonacich power centrality score; can be negative
- rescale
- if true, centrality scores are rescaled such that they sum to 1.
- tol
- tolerance for near-singularities during matrix inversion (see
`solve`

) - sparse
- Logical scalar, whether to use sparse matrices for the
calculation. The
Matrix package is required for sparse matrix support

##### Details

Bonacich's power centrality measure is defined by
$C_{BP}\left(\alpha,\beta\right)=\alpha\left(\mathbf{I}-\beta\mathbf{A}\right)^{-1}\mathbf{A}\mathbf{1}$, where $\beta$ is an attenuation parameter (set
here by `exponent`

) and $\mathbf{A}$ is the graph adjacency
matrix. (The coefficient $\alpha$ acts as a scaling parameter,
and is set here (following Bonacich (1987)) such that the sum of squared
scores is equal to the number of vertices. This allows 1 to be used as a
reference value for the ``middle'' of the centrality range.) When
$\beta \rightarrow$$1/\lambda_{\mathbf{A}1}$ (the reciprocal of the largest
eigenvalue of $\mathbf{A}$), this is to within a constant multiple of
the familiar eigenvector centrality score; for other values of $\beta$,
the behavior of the measure is quite different. In particular, $\beta$
gives positive and negative weight to even and odd walks, respectively, as
can be seen from the series expansion
$C_{BP}\left(\alpha,\beta\right)=\alpha \sum_{k=0}^\infty \beta^k$$\mathbf{A}^{k+1} \mathbf{1}$ which converges so long as $|\beta|$$< 1/\lambda_{\mathbf{A}1}$.
The magnitude of $\beta$ controls the influence of distant actors
on ego's centrality score, with larger magnitudes indicating slower rates of
decay. (High rates, hence, imply a greater sensitivity to edge effects.)

Interpretively, the Bonacich power measure corresponds to the notion that
the power of a vertex is recursively defined by the sum of the power of its
alters. The nature of the recursion involved is then controlled by the
power exponent: positive values imply that vertices become more powerful as
their alters become more powerful (as occurs in cooperative relations),
while negative values imply that vertices become more powerful only as their
alters become *weaker* (as occurs in competitive or antagonistic
relations). The magnitude of the exponent indicates the tendency of the
effect to decay across long walks; higher magnitudes imply slower decay.
One interesting feature of this measure is its relative instability to
changes in exponent magnitude (particularly in the negative case). If your
theory motivates use of this measure, you should be very careful to choose a
decay parameter on a non-ad hoc basis.

##### Value

- A vector, containing the centrality scores.

##### Note

This function was ported (ie. copied) from the SNA package.

##### Warning

Singular adjacency matrices cause no end of headaches for
this algorithm; thus, the routine may fail in certain cases. This will be
fixed when I get a better algorithm. `power_centrality`

will not symmetrize your
data before extracting eigenvectors; don't send this routine asymmetric
matrices unless you really mean to do so.

##### References

Bonacich, P. (1972). ``Factoring and Weighting Approaches to
Status Scores and Clique Identification.'' *Journal of Mathematical
Sociology*, 2, 113-120.

Bonacich, P. (1987). ``Power and Centrality: A Family of Measures.''
*American Journal of Sociology*, 92, 1170-1182.

##### See Also

##### Examples

```
# Generate some test data from Bonacich, 1987:
g.c <- graph( c(1,2,1,3,2,4,3,5), dir=FALSE)
g.d <- graph( c(1,2,1,3,1,4,2,5,3,6,4,7), dir=FALSE)
g.e <- graph( c(1,2,1,3,1,4,2,5,2,6,3,7,3,8,4,9,4,10), dir=FALSE)
g.f <- graph( c(1,2,1,3,1,4,2,5,2,6,2,7,3,8,3,9,3,10,4,11,4,12,4,13), dir=FALSE)
# Compute power centrality scores
for (e in seq(-0.5,.5, by=0.1)) {
print(round(power_centrality(g.c, exp=e)[c(1,2,4)], 2))
}
for (e in seq(-0.4,.4, by=0.1)) {
print(round(power_centrality(g.d, exp=e)[c(1,2,5)], 2))
}
for (e in seq(-0.4,.4, by=0.1)) {
print(round(power_centrality(g.e, exp=e)[c(1,2,5)], 2))
}
for (e in seq(-0.4,.4, by=0.1)) {
print(round(power_centrality(g.f, exp=e)[c(1,2,5)], 2))
}
```

*Documentation reproduced from package igraph, version 1.0.0, License: GPL (>= 2)*