Re: [igraph] why different result for the same data set

[Tamas Nepusz]

> I tried the power.law.fit in igraph first, and get the alpha = 1.85.  
> [...] However, when I use the plfit (http://www.santafe.edu/~aaronc/powerlaws/plfit.r 
> ) to calculate the alpha again, I got
> $xmin
> [1] 19
> $alpha
> [1] 2.19
> $D
> [1] 0.02631497.
Well, fitting a power-law distribution to empirical data is a really  
tricky problem (see http://arxiv.org/abs/0706.1062) and the result you  
get depends heavily on the method you use and the cutoff point. The  
method of Clauset et al determines the cutoff point automatically  
based on a Kolmogorov-Smirnov test (iirc), while igraph simply takes  
the lowest value as the cutoff point unless you specify otherwise and  
then it simply uses a maximum likelihood estimation. You can see that  
the method of Clauset et al determined the most appropriate cutoff  
point as xmin=19; maybe you can try and re-run power.law.fit using  
xmin=19 and see whether the results differ.

--
Tamas