igraph-help
[Top][All Lists]

## Re: [igraph] Walktrap / Community / Modularity

 From: Gábor Csárdi Subject: Re: [igraph] Walktrap / Community / Modularity Date: Fri, 6 Jul 2012 09:38:34 -0400

```Hi Amanda,

how exactly is vertex modularity defined? Do you have a reference?

Gabor

On Fri, Jul 6, 2012 at 4:47 AM, Amanda Schierz <address@hidden> wrote:
> Hi.
>
> Thanks for your reply. Do you know if there is a way I can calculate vertex
> modularity? I want to know how modular a vertex is - ie. does its edges stay
> within the community or cross communities - is it a hub of the community or
> is it the community/rest of graph communicator.
>
> thanks
> ________________________________________
> Sent: 05 July 2012 22:50
> To: Help for igraph users
> Subject: Re: [igraph] Walktrap / Community / Modularity
>
> Hi,
>
>> 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.
> No, they aren't. Modularity refers to the "quality" of a partition with
> respect to the whole network. The reason why you may be getting a vector is
> because walktrap is a hierarchical clustering method, i.e. it merges pairs of
> vertices or communities progressively, and the modularity vector returns the
> modularity of the partition after _each_ merge. The correlation you see is
> purely artificial: since merging pairs of vertices tends to increase the
> modularity of the partition (when the algorithm is doing its job properly and
> selects the right pair of vertices or communities in each step), the
> modularity score will increase for a long while, then have a peak somewhere
> (which one may use to select where to cut the dendrogram that the walktrap
> algorithm generates), then drop back to zero.
>
> Best,
> T.
>
>
> _______________________________________________
> igraph-help mailing list
> https://lists.nongnu.org/mailman/listinfo/igraph-help
>
> The Institute of Cancer Research: Royal Cancer Hospital, a charitable Company
> Limited by Guarantee, Registered in England under Company No. 534147 with its
> Registered Office at 123 Old Brompton Road, London SW7 3RP.
>
> This e-mail message is confidential and for use by the addressee only.  If
> message to the sender by replying to it and then delete the message from your
> computer and network.
>
> _______________________________________________
> igraph-help mailing list