[Ifeffit] Estimating error in XAS

Scott Calvin scalvin at anvil.nrl.navy.mil
Mon Jul 7 08:38:40 CDT 2003

Hi all,

Michel Schelgel said:

>I think an evaluation of the spectrum noise may be obtained
>from the FT contributions peaking at high 'distances' (e.g.,
>  > 10Å).

Although I think the scheme outlined by Michel is basically a good 
one, I would not advise using the high r FT as a measure of the 
spectrum's overall noise, at least on unfamiliar beamlines. It is my 
experience that on some beamlines there are spurious effects, such as 
those related to monochromator lock-in, pumps, etc. that introduce 
high-frequency (i. e. high r) oscillations without affecting 
oscillations in the 1-10 Angstrom range. The method IFEFFIT uses for 
estimating errors in fitted parameters is fortunately not dependent 
on this error estimate, although some statistical information, such 
as the value of chi-squared, is. But using high-r noise to weight 
spectra during merges may rely more about the behavior of a pump or a 
feedback mechanism than on the actual noise within the FT's region of 
interest. Unless I have a better way of estimating noise, I prefer to 
throw out obviously screwy scans and count the rest equally.

--Scott Calvin

More information about the Ifeffit mailing list