We propose a measure that ranks groups based on their interaction 
  with the rest of some predefined universe. The measure is easy to
  evaluate, and we explore possible applications. We also derive 
  formulas for the mean and variance of this measure that can be
  computed quickly and hence used effectively in an interactive data
  analysis environment. We outline a computational method for higher
  order moments of this measure, but this computation involves a 
  deeper analysis of the associated network structure.