R igraph manual pages

Assortativity coefficient

Description

The assortativity coefficient is positive is similar vertices (based on some external property) tend to connect to each, and negative otherwise.

Usage

assortativity(graph, types1, types2 = NULL, directed = TRUE)

assortativity_nominal(graph, types, directed = TRUE)

assortativity_degree(graph, directed = TRUE)

Arguments

graph	The input graph, it can be directed or undirected.
types1	The vertex values, these can be arbitrary numeric values.
types2	A second value vector to be using for the incoming edges when calculating assortativity for a directed graph. Supply NULL here if you want to use the same values for outgoing and incoming edges. This argument is ignored (with a warning) if it is not NULL and undirected assortativity coefficient is being calculated.
directed	Logical scalar, whether to consider edge directions for directed graphs. This argument is ignored for undirected graphs. Supply TRUE here to do the natural thing, i.e. use directed version of the measure for directed graphs and the undirected version for undirected graphs.
types	Vector giving the vertex types. They as assumed to be integer numbers, starting with one. Non-integer values are converted to integers with as.integer.

Details

The assortativity coefficient measures the level of homophyly of the graph, based on some vertex labeling or values assigned to vertices. If the coefficient is high, that means that connected vertices tend to have the same labels or similar assigned values.

M.E.J. Newman defined two kinds of assortativity coefficients, the first one is for categorical labels of vertices. assortativity_nominal calculates this measure. It is defines as

r=(sum(e(i,i), i) - sum(a(i)b(i), i)) / (1 - sum(a(i)b(i), i))

where e(i,j) is the fraction of edges connecting vertices of type i and j, a(i)=sum(e(i,j), j) and b(j)=sum(e(i,j), i).

The second assortativity variant is based on values assigned to the vertices. assortativity calculates this measure. It is defined as

sum(jk(e(j,k)-q(j)q(k)), j, k) / sigma(q)^2

for undirected graphs (q(i)=sum(e(i,j), j)) and as

sum(jk(e(j,k)-qout(j)qin(k)), j, k) / sigma(qin) / sigma(qout)

for directed ones. Here qout(i)=sum(e(i,j), j), qin(i)=sum(e(j,i), j), moreover, sigma(q), sigma(qout) and sigma(qin) are the standard deviations of q, qout and qin, respectively.

The reason of the difference is that in directed networks the relationship is not symmetric, so it is possible to assign different values to the outgoing and the incoming end of the edges.

assortativity_degree uses vertex degree (minus one) as vertex values and calls assortativity.

Value

A single real number.

Author(s)

Gabor Csardi csardi.gabor@gmail.com

References

M. E. J. Newman: Mixing patterns in networks, Phys. Rev. E 67, 026126 (2003) http://arxiv.org/abs/cond-mat/0209450

M. E. J. Newman: Assortative mixing in networks, Phys. Rev. Lett. 89, 208701 (2002) http://arxiv.org/abs/cond-mat/0205405/

Examples

# random network, close to zero
assortativity_degree(sample_gnp(10000, 3/10000))

# BA model, tends to be dissortative
assortativity_degree(sample_pa(10000, m=4))

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