Re: [Ifeffit] Linear combination fitting reports

Thanks for the fast answers. Attached is a screenshot. Maybe I just read the pages incorrectly. cheers, Jens ifeffit-request@millenia.cars.aps.anl.gov schrieb:

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Today's Topics:

1. Linear combination fitting reports (Jens Kruse) 2. Re: Linear combination fitting reports (Bruce Ravel) 3. Re: Linear combination fitting reports (Kelly, Shelly)

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Message: 1 Date: Wed, 2 Sep 2009 10:13:55 +0200 From: Jens Kruse Subject: [Ifeffit] Linear combination fitting reports To: Message-ID: <4A9E2943.5020408@uni-rostock.de> Content-Type: text/plain; charset="ISO-8859-15"; format=flowed

Hi,

I have only two short questions. Is it possible that values for chi-square and reduced chi-square given in the "Fit results" and the "Combinatorics" page are inverted within Athena? In the CSV report for all fits, which of these both columns ("chisqr", "chinu" ) is chi-square and which is reduced chi-square?

thanks, Jens

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Message: 2 Date: Wed, 2 Sep 2009 09:40:48 -0400 From: Bruce Ravel Subject: Re: [Ifeffit] Linear combination fitting reports To: XAFS Analysis using Ifeffit Message-ID: <200909020940.48150.bravel@bnl.gov> Content-Type: text/plain; charset="iso-8859-1"

On Wednesday 02 September 2009 04:13:55 am Jens Kruse wrote:

...

Hi,

I have only two short questions. Is it possible that values for chi-square and reduced chi-square given in the "Fit results" and the "Combinatorics" page are inverted within Athena?

I tried running a combinatorial LCF series just before writing this email and I think these numbers are reported correctly. If you think otherwise, you will need to show me an example.

Of course, the values for both R-factor and reduced chi-square are rather peculiar in Athena's LC fitting, as explained in the manual:

http://cars9.uchicago.edu/~ravel/software/doc/Athena/html/analysis/lcf.html

...

In the CSV report for all fits, which of these both columns ("chisqr", "chinu" ) is chi-square and which is reduced chi-square?

I don't think these are reported wrongly. As I discuss in the manual, it is hard to know how to evaluate chi-nu in a XANES fit. I use epsilon=1 (for want of a better number) and I use the number of points in the data as the number of independent measurements. There are typically of the order of 100 points in a XANES scan, so chi-nu tends to be about 2 orders of magnitude smaller than chi-square, which itself is very small due to the value of epsilon.

All of this just underscores the squishiness of the statistical parameters in this context. Interpreting LCF results requies the use of prior knowledge and a very conservative interpretation of what Athena reports.

B

-- Jens Kruse Institute for Land Use Faculty for Agricultural and Environmental Sciences Rostock University Justus-von-Liebig-Weg 6 18059 Rostock GERMANY Phone: +49(0)381-498 3190