Hello, same old question that emerges again: What formula is used by IFEFFIT to calculate the number of independent points? Does it use the formula 2DkDR/pi? or 2DkDR/pi +1? or 2DkDR/pi +2? And how does it approximate the resulting number to integer? Next: as far as I understand, the conversion from chi^2 to reduced-chi^2 involves dividing the first by the number of degrees of freedom. The latter is obtained by subtracting the number or floating parameters from the number of independent point right?. What if one fixes the value of one parameter, let's say S0^2 to unity? Does the number of parameters includes S0^2 in that case? We consider that it does not. Here is an example. We find with IFEFFIT (we use Dk= 10 (from 2 to 12), DR=5 (from 1 to 6). chi^2 = 87.1 reduced chi^2 = 3.7 parameters: E0, three distances and three sigma^2 we set S0^2 fixed to 1 R-factor: 1.15% The ratio between chi^2 and reduced chi^2 is 23.54, which is not an integer number as we would expect. Plus, this number does not correspond to Ninp -Nvar (31.83 - 5 = 26.83) What are we missing? Sorry if the question sounds silly to most of you. Stefano -- ____________________________________________ Stefano Ciurli Professor of Chemistry Laboratory of Bioinorganic Chemistry Department of Agro-Environmental Science and Technology University of Bologna Viale Giuseppe Fanin, 40 I-40127 Bologna Italy Phone: +39-051-209-6204 Fax: +39-051-209-6203 "Fatti non foste a viver come bruti, ma per seguir virtute e canoscenza" Dante Alighieri - Inferno - Canto XXVI "Ihr seid bestimmt, nicht Tieren gleich zu leben, Nein, Tugend zu erringen und Erkenntnis" "Ye were not form'd to live the life of brutes, But virtue to pursue and knowledge high"
On Wednesday 14 December 2005 10:21, Stefano Ciurli wrote:
Here is an example. We find with IFEFFIT (we use Dk= 10 (from 2 to 12), DR=5 (from 1 to 6).
chi^2 = 87.1 reduced chi^2 = 3.7 parameters: E0, three distances and three sigma^2 we set S0^2 fixed to 1 R-factor: 1.15%
The ratio between chi^2 and reduced chi^2 is 23.54, which is not an integer number as we would expect. Plus, this number does not correspond to Ninp -Nvar (31.83 - 5 = 26.83)
According to what you said above, Nvar = 7, so nu=24.83, which is much closer to 23.54. That still seems bigger than rounding errors, though. Here is my understanding of Ifeffit, but I am sure Matt can elaborate. Background removal uses the integer part of dk*dr*2/pi+1 to determine the number of knots. Fitting uses dk*dr*2/pi as a real number to determine Nidp. In any case, that's what Artemis reports when you ask it how many independent points in a project. My memory is that I checked it against Ifeffit's report of Nidp. B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advance 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/
Bruce,
According to what you said above, Nvar = 7, so nu=24.83, which is much closer to 23.54.
of course. My mistake. But still...
That still seems bigger than rounding errors, though.
that's what I am saying...
Here is my understanding of Ifeffit, but I am sure Matt can elaborate. Background removal uses the integer part of dk*dr*2/pi+1 to determine the number of knots. Fitting uses dk*dr*2/pi as a real number to determine Nidp. In any case, that's what Artemis reports when you ask it how many independent points in a project. My memory is that I checked it against Ifeffit's report of Nidp.
Not sure I fully follow, as I am not talking about background removal, but in any case, shouldn't the ratio between chi^2 and reduced-chi^2 be an integer number, whichever? Plus, I have found a few documents in which Matt reports Nidp as 2dkdr/pi, some others in which he reports 2dkdr/pi+1. In SK's thesis, which is available on the net (thanks Shelly :-)) she says +2. I was simply wondering which is the right formula used in IFEFFIT, and if it is different from what reported in Stern, Phys. Rev. B, 1993, 48, 9825-9827, why it is so. Stefano -- ____________________________________________ Stefano Ciurli Professor of Chemistry Laboratory of Bioinorganic Chemistry Department of Agro-Environmental Science and Technology University of Bologna Viale Giuseppe Fanin, 40 I-40127 Bologna Italy Phone: +39-051-209-6204 Fax: +39-051-209-6203 "Fatti non foste a viver come bruti, ma per seguir virtute e canoscenza" Dante Alighieri - Inferno - Canto XXVI "Ihr seid bestimmt, nicht Tieren gleich zu leben, Nein, Tugend zu erringen und Erkenntnis" "Ye were not form'd to live the life of brutes, But virtue to pursue and knowledge high"
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Bruce Ravel -
Stefano Ciurli