1. A computer program product comprising a non-transitory tangible machine-readable medium that stores a program, the program being executable by a machine to perform a method of identifying a measure of similarity between nodes of first and second networks, the method comprising: identifying a node pair (i, j), where i is a node from the first network and j is a node from the second network, and where u is a neighbor of i and v is a neighbor of j; and computing a network similarity score R ij the node pair to be equal to a total support provided by all supporting node pairs adjacent to the node pair, each supporting node pair providing its support in proportion to a number of supporting node pairs it has to support; where the first and second networks each represent a physical system selected from a set of physical systems, the physical system being one of: a protein interaction network, a digital data set.