[Ifeffit] R-factor

Alison L Costello acostell at unm.edu
Wed Jul 6 10:11:47 CDT 2005


I've looked, and my R-factors for the back-transformed 
space are not necessarily twice the value of the R-factors 
for the k-space fits.  In fact, they are often quite 
close.  For example, just recently I had a k-space 
R-factor of 0.102 and a q-space R-factor of 0.113.  Now, I 
realize those numbers are very close, but I'm afraid if I 
try to publish this, then I will get criticism for the 
q-space R-factors being larger.  If I can explain it, then 
maybe it won't be a problem.

I do see what you're saying about the complex vs. real 
function, and that definately makes sense, but they don't 
differ by a factor of 2.  Hmmm.


On Wed, 6 Jul 2005 09:43:03 -0500
  Bruce Ravel <bravel at anl.gov> wrote:
> On Tuesday 05 July 2005 18:39, Alison Costello wrote:
>> I have a question regarding the calculation of the 
>>R-factor.  I am
>> currently using SIXPACK to process my protein EXAFS 
>>data, and I fit the
>> data in both k-space and in q-space (back-transformed 
>>k-space).  I
>> typically will use k-ranges = [1-13.6] and for 
>>multiple-scattering fits,
>> will use an R-range of 0.1 - 4.5.  Sometimes, the 
>>R-factor for the q-space
>> fit is greater than the R-factor for the k-space fit, 
>>which should not be
>> true, as the back-transformed space filters out noise. 
>> I am confused as to
>> why this occurs, and am wondering if the R-factor for 
>>the back-transformed
>> space is calculated differently than for k-space fits.
> Alison,
> I would expect the R factor to be bigger in k space due 
>to the high frequency
> portions of the data that get filtered out of the q 
>space data.  That is what
> happened in the one example I just tried.
> What are some example values of the R-factors?  Do they 
>differ by about a
> factor of 2?
> One thing that occurs to me is that chi(q) is a complex 
>function while chi(k)
> is a real function.  In Ifeffit those two functions 
>should have the same
> number of points, but the R factor in q is computed 
>using both parts of the
> complex function.  Thus it would seem reasonable if they 
>differed by about 2
> and there were no significant Fourier components in the 
>data beyond your rmax.
> B
> -- 
> Bruce Ravel  ----------------------------------- 
>bravel at anl.gov  -or-
> 						  ravel at phys.washington.edu
> Environmental Research Division, Building 203, Room 
> Argonne National Laboratory 
>                       phone: (1) 630 252 5033
> Argonne IL 60439, USA 
>                               fax: (1) 630 252 9793
> My homepage:    http://feff.phys.washington.edu/~ravel 
> EXAFS software: 
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