Network homophily via tail inequalities
Homophily is the principle whereby "similarity breeds connections". We give a quantitative formulation of this principle within networks. Given a network and a labeled partition of its vertices, the vector indexed by each class of the partition, whose entries are the number of edges of the subgraphs induced by the corresponding classes, is viewed as the observed outcome of the random vector described by picking labeled partitions at random among labeled partitions whose classes have the same cardinalities as the given one. In this perspective, the value of any homophily score $Θ$, namely a non decreasing real valued function in the sizes of subgraphs induced by the classes of the partition, evaluated at the observed outcome, can be thought of as the observed value of a random variable. Consequently, according to the score $Θ$, the input network is homophillic at the significance level $α$ whenever the one-sided tail probability of observing a value of $Θ$ at least as extreme as the observed one, is smaller than $α$. Since, as we show, even approximating $α$ is an NP-hard problem, we resort to classical tails inequality to bound $α$ from above. These upper bounds, obtained by specializing $Θ$, yield a class of quantifiers of network homophily. Computing the upper bounds requires the knowledge of the covariance matrix of the random vector which was not previously known within the random coloring model. In this paper we close this gap, giving a meaningful, easy to compute class of indices for measuring network homophily. As demonstrated in real-world network applications, these indices are effective, reliable, and lead to new discoveries that could not be captured by the current state of the art.