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[igraph] Walktrap / Community / Modularity

From: Amanda Schierz
Subject: [igraph] Walktrap / Community / Modularity
Date: Thu, 5 Jul 2012 16:13:27 +0100


I have a network with approx 9500 vertices. The vertices are labelled from 0 
upwards. I ran Walktrap.communities and have the membership vector. I then ran 
Modularity on the walktrap object and a vector was returned the same length as 
the number of vertices. I therefore assume these are vertex modularity scores.

I don't know if I have misunderstood modularity but there is a direct 
correlation between the modularity score and the vertex ID. If I sort the 
vertex modularity scores in ascending order then it's 98% the same as the 
vertex ordering (I thought there might be some ordering bias because of the use 
of clustering). Very significant p value between modularity and vertex id.

Have I made a big error in my understanding of communities and modularity? Also 
for vertex id 0 (which is needed for igraph) the modularity is 0 (in the 
modularity documentation it says that communities should start from 1 but 
walktrap starts the communities from 0)  The is no pattern between the 
community and the vertex ID.

many thanks for your help,


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