To introduce the topic, we require the language of graph theory. For a group of n subjects, define a graph G where {1, 2, …, n} is the vertex set (composed of subjects in the study). The graph G can be associated with a weight matrix W, that reflects the strength of the connections between pairs of subjects: the higher the value of the entry wij, the stronger the connection between the pair (i,j). Edges that are not connected have weight 0. The weight matrix W is symmetric with non-negative entries.
