[Ifeffit] Modelling texture in Artemis

Matt Newville newville at cars.uchicago.edu
Wed Aug 15 10:48:20 CDT 2018

Hi Joselaine,

On Tue, Aug 14, 2018 at 10:21 AM Joselaine Cáceres gonzalez <
joselainecaceres at gmail.com> wrote:

> Dear all,
> I am new in this group so I don't know if this is the right place to ask
> about it, but I´d like to know about LCF performed with Athena in XANES
> data. I had some difficulties in founding information about errors reported
> by the program.
> I am working with different techniques and in some cases I have to
> calculate uncertainty in results reported. In LCF results reported by
> Athena I don't know how to interpret the errors reported between
> parenthesis along with each model spectra.
> In my reports I carefully define if I am reporting the standard deviation
> of the mean, a combined standard uncertainty,etc. depending each case, the
> coverage probability, etc. So I need to understand the information Athena
> is reporting as errors in LCF.

The reported uncertainties are the 1-sigma standard errors.  However, the
uncertainties in the data are necessary to get the scale of the standard
errors correctly, and these uncertainties in the data are (almost always)
not supplied for the analysis, so the program has to guess what those
uncertainties should be.

To do this it follows a common if not-quite-rigorous approach of first
asserting that the fit is "good" (otherwise you probably wouldn't care
about the detailed scale of the uncertainties) so that reduced chi-square
should be 1.   That is, it effectively rescales the uncertainty in the
data, epsilon, so that reduced chi-square is 1, and uses that to scale the
reported uncertainties. Another way to look at this is that the reported
uncertainties increase the reported chi-square by reported reduced
chi-square, without regard to the value of epsilon used.

Hope that helps.  Please try to use a separate topic thread in the future,
as this question has little or nothing to do with the subject line...

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