[Ifeffit] doubt on number of variables

Jesús Eduardo Vega Castillo jevecas at gmail.com
Thu May 26 09:37:56 CDT 2016


Thank you very much Bruce and Scott,

Your answers are really helpful.

2016-05-26 11:28 GMT-03:00 Bruce Ravel <bravel en bnl.gov>:

> On 05/26/2016 10:02 AM, Jesús Eduardo Vega Castillo wrote:
>
>> Just for clarifying, this is not what I have been doing. I just was
>> asked to add new independent variables (individual DW factors for each
>> path) while I am already at the limit of independent points.
>>
>> I have been using a number of parameter always lower than the number of
>> independent points. I have also been managing high correlations by not
>> varying two strongly correlated parameters at the time. But at the end I
>> do a final fit setting to guess all the parameters, no matter the
>> correlations, in order to obtain a "true" reduced Chi2 to report which
>> includes all parameters. Do you consider this procedure right?
>>
>
> Well, "truth" is a difficult concept and isn't really what EXAFS analysis
> is all about.
>
> When we do an EXAFS analysis we are testing the extent to which data are
> consistent with a fitting model.  The goal is to find a fitting model that
> is a defensible description of the data and which yields fitting results
> and uncertainties that are, themselves, defensible and which tell us
> something that we want to know about the sample which was measured.
>
> Some parameters are highly correlated and there is nothing you can do
> about it, regardless of how much you might want to.  In a one-shell fit,
> S02 and coordination number are 100% correlated.  It doesn't matter how
> much your thesis advisor wants you to get the "true" coordination number
> from the one-shell fit, it won't happen.
>
> So, in the end, you settle upon a fitting model and write a paper.  In
> that paper you report on the fitting results.  If the referee asks why you
> chose to fix a certain parameter, you better have a good reason. That's
> what I mean by "defensible" -- that you can explain and justify all the
> choices you made when fitting the data.
>
> The "procedure" is that you define a fitting model, press the big Fit
> button, and interpret the results.  The parameters that you float, the
> parameters that you fix ... they are defensible when you can defend them.
> "Because the boss wanted two more sigma^2 parameters" is probably not a
> defensible argument.
>
> Here are a couple of my papers:
>
> http://dx.doi.org/10.1016/j.radphyschem.2009.05.024
> http://dx.doi.org/10.1107/S1600577514014982
>
> I refer you to these not because they are necessarily relevant to your
> work or even because they are especially good papers.  But, in each, I
> discuss defensibility of fitting model in a situation where there is not
> nearly enough information in the measured data to properly describe the
> actual structure of the sample.  Perhaps you'll find it helpful to see how
> I address this issue.
>
>
> B
>
> --
>  Bruce Ravel  ------------------------------------ bravel en bnl.gov
>
>  National Institute of Standards and Technology
>  Synchrotron Science Group at NSLS-II
>  Building 535A
>  Upton NY, 11973
>
>  Homepage:    http://bruceravel.github.io/home/
>  Software:    https://github.com/bruceravel
>  Demeter:     http://bruceravel.github.io/demeter/
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