I use a formula from Newman/Girvan (Phys. Rev. E 69 (2004) 026113) to calculate Q=SUMi(eii-ai*ai) with ai=SUMj(eij) from the k x k symmetric matrix e. eii is the fraction of links
within the cluster k and ai is the row (or column) sum. Very easy to implement in standard statistical packages. Here are my questions:
1. Why has matrix e to be symmetric?
2. Why can ai*ai be characterized as an appropriate null model?
(Both are questions from a reviewer ;-))
Best regards to the specialists
Dr. Per Kropp
Institute for Employment Research
Regional Research Network
Frau-von-Selmnitz-Str. 6, D-06110 Halle