The degree is defined as the number of neighbours that a node has and is most often denoted as k. In directed graphs, the degree has two components, the number of incoming edges k in , (in-degree), and the number of outgoing edges k out , (outdegree). The degree distribution is defined as the set of probabilities, P(k), that a node chosen at random will have degree k. Plotting the distribution of degrees of nodes is one of the most basic and important ways of characterising a given network (Figure 2 ). In addition, useful characterisations are obtained by calculating the moments of the degree distribution. The nth moment of P(k) is defined as