[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
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