[Ifeffit] XANES fitting

[Bruce Ravel]

On Wednesday, October 12, 2011 02:35:21 pm Matt Newville wrote:

> > I used to quote in my papers that typical statistical and
> >  systematic errors lead to a total error in first shell distances
> >  of +-0.02 A.
> Quoting +/- 0.02 Ang seems like a reasonable estimate for a
> typical value, but that's hardly rigorous.

I don't have a better answer for Riti's question than the one Matt
gave.

I do, however, want to get up on a soapbox about the practice of
quoting typical error bars.  While I certainly agree that something
like 0.02 is an unsurprising uncertainty in a distance, there is a
very good reason to always report measured uncertainties.

The problem with sweeping measured uncertainties under the rug by
asserting a typical value is that it is probably not true that for a
real-world, interesting, research project that *all* the uncertainties
are of a typical size.  Because of issues of experimental error and
correlation between parameters, it is likely that some of your
parameters have uncertainties that are larger than the typical value.
You may even have some that are smaller.

It is probably also true that some of the most interesting parameters
in the fit -- the ones that are most salient to the argument being
made in the manuscript -- are the ones with non-typical uncertainties.
If you fail to report that part of your analysis, you are depriving
your reader of information that is important to the evaluation of your
results.

It is for that reason that I discourage the practice of citing typical
uncertainties.

B

P.S. This paper by Corwin Booth about error analysis was one of my
favorite presentations at the last XAFS conference
   http://dx.doi.org/10.1088/1742-6596/190/1/012028


-- 

 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