[Ifeffit] ifeffit R-factor

[Matt Newville]

Hi Sandra,

On Sat, Apr 6, 2013 at 12:37 PM, Sandra Luber <s.luber at pci.uzh.ch> wrote:
> Dear Matt Newville,
>
> I do some fitting with ifeffit using EXAFS data generated by feff.
> I wonder how the R-factor is calculated. Unfortunately,
> I have not found any definition yet. Would it be possible
> that you write me how it is obtained?
> This would be great.
>
> Thanks a lot.
>
> Best regards
> Sandra Luber
>

The basic definition is
   R = Sum(  |data - fit|^2 ) /  Sum( |data|^2 )

The old Feffit document probably has the clearest definition, in its
Chapter 5 (pages 16-20 of
http://cars.uchicago.edu/~newville/feffit/feffit.pdf ).

The definition and briefer explanations are also given in several of
the tutorials on http://xafs.org/Tutorials.

Thanks for reminding me to put this and related information into the
Larch documentation!

--Matt