[Ifeffit] Chi^2 and R-factor reporting requirements

[Scott Calvin]

Hi Stanislav,

I'm glad you brought this up--I'm very interested in hearing a debate 
on this issue.

Here's my two cents: in principle, I think reporting a reduced chi^2 
is a good idea. The problem, of course, is that a meaningful reduced 
chi-square requires a meaningful estimate of the measurement error 
epsilon. The default behavior of Ifeffit is to use the noise in the 
high-R fourier transform (I believe it uses 15-25 Angstroms) for 
epsilon. Ifeffit also allows you to override this value if you think 
you have a better estimate.

In some cases, the default ifeffit estimate for epsilon is probably 
fairly poor. I have used a beamline, for example, which often showed 
high-R oscillations (due, perhaps, to feedback problems). These 
high-R oscillations had little or no effect on the EXAFS, but, by 
increasing Ifeffit's estimate of epsilon, resulted in a lower reduced 
chi^2 than if I had recorded the data on another beamline!

Because of this, I ignore published reduced chi^2's that don't 
include an argument as to why the epsilon they chose (or let Ifeffit 
choose) is a reasonable value. I would generally prefer such reduced 
chi^2's not be included in the article.

Having said that, I'd love a discussion as to how people obtain 
meaningful estimates of epsilon, or under what conditions they 
consider the Ifeffit default good enough.

As you noted, it is worth reiterating that the algorithm Ifeffit uses 
to find uncertainties in fitted parameters is independent of the 
choice of epsilon.

--Scott Calvin
Sarah Larence College

>Hello all,
>
>I would like to hear your opinion about reporting Chi^2 and R-factor
>values in EXAFS studies.
>
>I found that Chi^2 and R-factor values are not provided by many EXAFS
>papers.
>However, according to standards of The International XAFS Society
>(http://ixs.iit.edu/) both these values should be reported.
>
>My understanding is that the reduced Chi^2 should be close to 1 in ideal
>case. In practice however, the value is always greater than 1 because of
>systematic errors.
>For example, my reduced Chi^2 values are around 300.
>However, the R-factor values are about 3% which doesn't look as "ugly" as
>Chi^2 reduced.
>
>On the other hand, ifeffit overestimates uncertainties using reduced Chi^2
>value. Thus, all errors are taken into account.
>The final question: Is it appropriate to report just R-factor calculated
>by ifeffit and the obtained uncertainties (which look reasonable) without
>reporting the "ugly" Chi^2/reduced Chi^2 values?
>
>Stanislav