On Thursday 04 January 2007 03:06, Sang-Wook Han wrote:
Hi! Bruce
I have a question of the independent data points. I calculate the indenpent data points using Stern's idea, N = 2(Kmax - Kmin)*(Rmax-Rmin)/3.14 + 2. As you can see the attached file, the result of the Artemis is different from this method. How do you determine the independent data points?
Hi Han, I am taking the liberty of CCing my response to the Ifeffit mailing list since this is a very common question and quite approriate for discussion in that forum. The issue of enumerating independent points is one that has been discussed at some length already on the mailing list. Here are some links. In this one, Matt summary is quite consistent with my view on this issue: http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2005-December/002222.html Here Scott makes reference to a paper applying Baysing approaches to EXAFS data analysis and to the fact that the EXAFS measurement is not, in fact, an ideally packed signal. http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2005-December/002221.html It seems to me that if your analysis requires the "+2", then you are almost certainly placing excessive demand on your data. Almost certainly, you are using parmaters that are highly correlated and are seeing quite large error bars. If that is the case, you will need to think about how to use constraints to simplify your fitting problem. See question 3 at http://cars9.uchicago.edu/iffwiki/FAQ/FeffitModeling. Regards, B P.S. It would be very helpful if someone could make am entry on that same FAQ page discussing the issue of enumerating Nidp. -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/