Pearson's correlation coefficient in the theory of networks: A comment

In statistics, the Pearson correlation coefficient rx,y determines the degree of linear correlation between two variables and it is known that -1 ≤ rx,y ≤ 1. In the theory of networks, a curious expression proposed in [PRL bf 89 208701 (2002)] for degree-degree correlation coefficient rji,ki, i∈ [1,M] has been in use. We realize that the suggested form is the conventional Pearson's coefficient for (ji,ki), (ki,ji) for 2M data points and hence it is rightly dedicated to undirected networks.