Ok. Thanks Tamas. Tada
On Wed, May 27, 2015 at 9:18 PM, Tamas Nepusz <[email protected]> wrote: > Hi, > > > I'm trying to calculate transitivity of the form B <- A -> C, where also > > B->C in a directed graph. For this I counted all the neighbors looping > over > > each vertex using the function neighbors(graph[i], mode='out'). Then I > > counted the number of neighbors using the function length. Finally I get > > the number of possible triplets for each vertex using combinatorial, i.e. > > choose(number.neihgbors, 2) and I got the number of triplets of the > form B > > <- A -> C for each vertex. When I added them all and then compare with > the > > results from triad census for the same kind of triplets results didn't > > match. Any idea why? > You have calculated the number of (A,B,C) triplets where the following > conditions hold: > > - there is an edge from A to B > - there is an edge from A to C > > Note that we do not know whether there is an edge from B to A, C to A, B to > C or C to B. However, when you look at the number of out-star (i.e. B <- A > -> > C) triads in the triad census, this would give you the number of those > triads, > where there is an edge from A to B and A to C but there is _no_ edge from > B to > A, B to C, C to A or C to B. So that's why the two numbers are different. > > (Footnote: you can simply do this -- it should be equivalent to what you do > now: > > degs <- degree(g, mode="out") > sum(degs*(degs-1)/2) > ) > > As for calculating the transitivity you need, basically you need the ratio > of > the number of triplets that fit the (B <- A -> C, B -> C) pattern, divided > by the > number of triplets that fit the (B <- A -> C) pattern. The numerator and > the > denominator will be a sum of certain components of the triad census (see > ?triad.census for the exact meaning of each component), you only need to > figure > out which of the triads fit the two patterns. > > T. > > _______________________________________________ > igraph-help mailing list > [email protected] > https://lists.nongnu.org/mailman/listinfo/igraph-help > -- *Tadaishi Yatabe* DVM, MPVM, PhD (C) Center for Animal Disease Modeling and Surveillance (CADMS) Department of Medicine and Epidemiology School of Veterinary Medicine University of California Davis
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