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