
From:  Tamas Nepusz 
Subject:  Re: [igraph] betweenness centrality  negative and floating values? 
Date:  Thu, 11 Jun 2009 00:52:54 +0100 
Hi Yong,I tested your graph and it looks like the issue is platformdependent; e.g., it doesn't happen in openSuSE 11.1 (gcc 4.3.2, igraph 0.5.2), but I can reproduce it with OS X (gcc 4.0.1, igraph 0.5.2 and igraph 0.6). I suspect some kind of an internal overflow problem, but it takes more time to track down the bug than I expected. I filed a bug report on Launchpad; if you want to get notified when there's some progress, subscribe to the bug report here:
https://bugs.launchpad.net/igraph/+bug/385747  Tamas On 2009.06.10., at 11:43, Yong Zou wrote:
Hi Tamas, Could you please help me to understand those extremely big values for the betweenness centrality? 58743768046350440071168.000000 for a network of 10000 nodes. Thank you very much. YongOn Thu, May 28, 2009 at 1:38 PM, Yong Zou<address@hidden> wrote:Thanks Tamas,Now it is clear for those floating values. But what about the very bigor even negative values, then? Best, YongOn Thu, May 28, 2009 at 1:33 PM, Tamas Nepusz <address@hidden> wrote:Ok, I can understand the floating values for the BC then because ofthis weight of 1/k. It also means that the more shortest paths betweenA and B, the less weight will be counted. However from the original definition for BC,http://en.wikipedia.org/wiki/Centrality, this sounds to be differentfor me.No, it isn't. The formula on that Wikipedia page has the number of shortest paths between s and t in the denumerator, so if there is more than a single shortest path between s and t, the vertices participating in those paths will receive a contribution of 1/k due to this path, where k is \sigma_{s,t} the number of shortest paths between s and t.I'm quite confused by this now. The values should be normalized to the interval [0 1]?It can be normalized if you wish, but igraph doesn't do that by default. As it says on the Wikipedia page you mentioned, "This may be normalised by dividing through the number of pairs of vertices not including v, which is (n  1)(n  2) / 2.". So if you want to get values between 0 and 1, dividethe betweenness centrality values by (n1)(n2)/2.  Tamas _______________________________________________ igraphhelp mailing list address@hidden http://lists.nongnu.org/mailman/listinfo/igraphhelp_______________________________________________ igraphhelp mailing list address@hidden http://lists.nongnu.org/mailman/listinfo/igraphhelp
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