[Ifeffit] background subtractions
scalvin at anvil.nrl.navy.mil
Fri May 16 10:06:03 CDT 2003
My two cents, FWIW:
I prefer not to use the background found by Artemis in final fits
(well, actually I'm still using FEFFIT, but I think the issues Bruce
discussed haven't changed).
The correlations reported by IFEFFIT or FEFFIT, while very useful,
only really probe the area of fitting space right around one pair of
variables. In complex fits, there are often multiple minima in
fitting space, or nonlinear effects that involve more than one set of
variables. Just because a background subtracted during the fitting
process doesn't report a high correlation doesn't necessarily mean
that it hasn't influenced the fit. I came up again this issue when I
was doing multiple-dataset fits on a temperature series; if I let the
background refine in this way for the spectrum at each temperature, I
sometimes destroyed temperature trends in things like lattice
parameters that showed up perfectly well without the second
background subtraction...and the effect of the background did not
show up in the reported correlation coefficients.
In terms of the "not cheating" argument (the number of knots is the
same as the number of independent points), I need to be convinced
that it is really OK on data that has already been through Athena.
After all, haven't two splines been subtracted from the data? So why
doesn't that gobble up twice as many of the independent points?
Currently, I do use the background feature during the fitting
process, because it does help guide me as to where rbkg should go in
the way Bruce describes. But then I run AUTOBK (basically the routine
used by Athena) with the rbkg at the value I've determined is
appropriate and leave it at that. That way, I'm comfortable that rbkg
has been chosen in some rational, defensible way (the big selling
point for using Artemis to further refine the background), but I
don't have to think about what it means to use a two-pass background
Naval Research Lab
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