[Ifeffit] number of independent points

Matt Newville newville at cars.uchicago.edu
Mon Oct 10 10:34:16 CDT 2011

Of course. I do agree with both Jeremy and Bruce.
On Oct 10, 2011 10:07 AM, "Bruce Ravel" <bravel at bnl.gov> wrote:

> On Monday, October 10, 2011 10:48:25 am Kropf, Arthur Jeremy wrote:
> > Perhaps there is a more subtle analysis of the data range, but I doubt
> > it, since Matt is on record as suggesting that people not place too much
> > emphasis on the exact value of N-idp, if I recall correctly.  You can
> > scale your chi-square, reduced chi-square, and errors using whatever
> > value of N-idp you prefer.
> The papers by Rossner and Krappe (for instance,
> http://dx.doi.org/10.1103/PhysRevB.66.184303) use a Bayesian approach
> to evaluate the actual information content in an EXAFS signal.
> Unsurprisingly, the Bayesian analysis finds something we should
> already know -- the EXAFS signal is not an ideally packed sum of sine
> waves.  Consequently, the actual information content is rather less
> than the Nyquist criterion.
> That is the context of the suggestion not to place to much emphasis on
> the Nyquist number.  (Since Jeremy invoked Matt, I should say that I
> suspect that Matt would more or less agree with what I am saying
> here.)   It gives us a number to work with in evaluating the statistics
> in Ifeffit.  The interpretation of Ifeffit's statistical analysis
> (which is a simple Gaussian analysis for a problem that is ill-posed
> in the Guassian sense) has been a frequent topic of conversation on
> this mailinglist over the years.  It is also a topic in this talk that
> I sometimes give: https://github.com/bruceravel/XAS-
> Education/downloads/atea.pdf
> All that said, the question of the formula used and the value reported
> in Artemis is a valid one.  I don't recall off the top of my head,
> but I will look at the codes.
> B
> --
>  Bruce Ravel  ------------------------------------ bravel at bnl.gov
>  National Institute of Standards and Technology
>  Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
>  Building 535A
>  Upton NY, 11973
>  My homepage:    http://xafs.org/BruceRavel
>  EXAFS software:  http://cars9.uchicago.edu/ifeffit/Demeter
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