Hi Scott and Shelly, Thanks for the replies.
Scott, I have read a bit about how ifeffit computes the error. I have a hard time keeping that information handy when I start seeing stuff like this, but that was the source of my comment about it being related to compared chi-square with different k-ranges. Thanks for the refresher, hopefully it sinks in a bit better this time! I tried your suggestion with epsilon and the chi-square values came out to be very similar values with the different windows. Does this mean that reporting reduced chi-square values in a paper that compared several data sets would not be necessary and/or appropriate? Would setting a value for epsilon allow comparisons across different k-ranges, different (but similar) data sets, or a combination of the two using the chi-square parameter? Shelly, thanks for the tip on appropriate dk values for the two windows. I would appreciate a copy of your book chapter. Changing the dk value to 2 using the K-B window produced similar chi-square values compared to a Hanning window with a dk=1. In playing around with different windows and dk values my fit variables generally stayed within the error bars, but the size of the error bars could change more than a factor 2. Does this mean that it would make sense to find a window/dk that seems to "work" for a given group of data and stay consistent when analyzing that data group? Best Regards, Brandon On Wed, May 11, 2011 at 2:41 PM, Shelly Kelly <dr.sdke...@gmail.com> wrote: > Hi Brandon: > > I have a different idea on what might be happening. Take a look at the > window function and the data in k-space for both window functions. The > kaiser-bessel window works in a different way from the Hanning window. They > emphasize different regions of the data differently. The sill part of the > window is defined differently. Usually use a dk = 1 or 2 A-1 with a Hanning > window. Take a look at the kq data showing the signal q that is used in R > for the fit. The Kaiser-Bessel window works best with dk values of 3 to 4 > A-1. This results in a slowly giving more importance to data in the k-range. > Again look at the kq data to see this effect. > > If you use a Kaiser-Bessel window with a small dk then the data used in the > fit will extend more (as compared to Hanning) beyond the k-range set by kmin > and kmax. Same for the Hanning window, but with a large dk value as > compared to K-B. > So by simply changing the window without changing dk, you are changing the > information that is Fourier transformed and that results in slightly > different values and all the rest. > > If you need an example: I can send you my book chapter :). It has an > excellent section on FT and all the buttons that go with them. > > Kelly, S. D., Hesterberg, D. and Ravel, B. (2008). Analysis of soils and > minerals using X-ray absorption spectroscopy. Methods of soil analysis, Part > 5 -Mineralogical methods. Ulery, A. L. and Drees, L. R. Madison, WI, USA, > Soil Science Society of America: 367-463. > > Cheers, > Shelly > > On Wed, May 11, 2011 at 2:46 PM, Brandon Reese <bjre...@gmail.com> wrote: > >> Hello everybody, >> >> I am working on fitting some EXAFS of amorphous materials and have noticed >> an odd (in my mind) behavior when changing transform windows. I settled on >> a fit using all three k-weights and the Hanning transform window obtaining >> statistical parameters of R=0.0018 and chi_R=361. I decided to change the >> transform window to a Kaiser-Bessel to see what would happen. The refined >> parameters came out more or less the same, well within the error bars, with >> the Hanning windows having slightly smaller error bars. But my statistical >> parameters changed significantly to R=0.0022 and chi_R=89.354. It seems >> that this large change may be related to why we can't use the chi_R >> parameter to compare fits over different k-ranges, but I am not sure about >> that. Have other people seen this? I would guess it means that when >> looking for trends in different data sets, it is more important to be >> consistent, rather than which specific window type is used. >> >> Thanks, >> Brandon >> >> _______________________________________________ >> Ifeffit mailing list >> Ifeffit@millenia.cars.aps.anl.gov >> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >> >> > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > >
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit