A community in a network is a group of vertices such that there is a higher density of edges within the group than between them.
Given a graph , let the vertex set V be partitioned into k subsets such that each subset belongs to one community. The community modularity Q of this partition is defined as
where is the percentage of number of edges that has both ends in community , and is the percentage of edges that start from community . In other words,
and
.
The community modularity Q is a number less than or equal to 1. A large positive value indicates that the vertex partition gives significant community structure.