Re: [Ifeffit] background subtractions

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 subtraction.