Thanks again Bruce.  Yes that helps.  What I want to do is defend the criticism of my fits by honestly assessing their quality, i.e., not over stating the results based on fits that compromise accuracy for precision.  As you describe in the 2009 RP&C paper on mercury binding to DNA, when dealing with a limited number of independent data points yet several paths that contribute significantly to the EXAFS signal, logically creative methods to constrain fitting parameters are necessary.

 

I have found the method that Matt Newville describes in that thread to be a useful method during the data analysis process.  However, I have found it to be very important to re-open the fit to both shells after fitting the second shell.  I use this method of fitting the second shell while fixing the first shell to filter out models of similar first and second shell structure yet different second shell composition, e.g., a mixed metal versus a single metal in the second shell.  For both fitting models, the first shell is very similar (e.g., circa 6 oxygens).  If both models have similar first shell values and correlations, perhaps it is reasonable to ignore them during part of the fitting routine to focus on the small details of the second shell that can get lost in the more dominant features of the first shell.  Essentially, I compare the RCS values of the second shell fits of two models that have the same first shell to choose which model is better.  I will test varying some of the procedural parameters in the fit (k-weighting, k and R ranges) as you suggested.  I’ve seen some discussion about this on the mail list, and it is also strongly recommended there.

 

For those interested in F-tests, I’ve found the following resources helpful:

 

Title: F-test in EXAFS fitting of structural models. Author(s): Michalowicz, A; Provost, K; Laruelle, S; et al. Source: JOURNAL OF SYNCHROTRON RADIATION  Volume: 6   Pages: 233-235   DOI: 10.1107/S0909049599000734 Part: Part 3   Published: MAY 1 1999

 

Title: Statistical evaluations in fitting problems. Author(s): Klementev, KV. Source: JOURNAL OF SYNCHROTRON RADIATION  Volume: 8   Pages: 270-272   DOI: 10.1107/S0909049500015351   Part: Part 2   Published: MAR 2001

 

Title: A Variation of the F-Test for Determining Statistical Relevance of Particular Parameters in EXAFS Fits. Author: Downward, L. Booth, C.H. Lukens, W.W. Bridges, F.

Publication Date: 07-25-2006 Publication Info: Lawrence Berkeley National Laboratory Permalink: http://www.escholarship.org/uc/item/5p60c864

 

And also here on the mailing list:

http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2009-August/008983.html

 

In the Michalowicz paper I’ve found values slightly different when reproducing his numbers.  I use some okay free web-based F-test calculators or excel – but if anyone has a preferred web-based F-test calculator that information would also be appreciated.

 

Kind regards,

Matt S.