[Ifeffit] Question about transform windows and statistical parameters

Matt Newville newville at cars.uchicago.edu
Thu May 12 09:40:32 CDT 2011


Scott's answer assumed that epsilon was, in fact, the source of
difference between the ratio of R and reduced chi-square, while
Shelly's answer assumed that the k-ranges of the window functions was
dramatically different.   Either (or both) of these could be part of
the explanation.     I would ask: Were the epsilons reported from the
two fits very different?, and  What were the full set of parameters
for the Fourier window function?

It appears from your response that epsilon was fairly different.
But you also say that you used a Kaiser-Bessel with dk = 1 or 2.
Yikes.  These give very poor FT windows.  The Kaiser Bessel window
should have dk of at least 3 (4 or 5 would be my recommendation), or
there is far too sharp a drop at kmin and kmax and much ringing of the
chi(R) data.    My suspicion would be that this is actually the main
cause of the original differences you were seeing.

Setting epsilon by hand is a completely reasonable thing to do, if you
have a better idea of the noise in chi(k) than Ifeffit's guess.
Pulling a number out of thin air might not qualify as a better
estimate. ;).    Setting epsilon to be the same for several data sets
could be OK, but it assumes that the spectra are equally noisy.
Again, this may be OK, but if you have to do this because Ifeffit's
estimates vary between data sets, than I'd suspect that they are not
equally noisy.

As Bruce says, the reported reduced chi-square is the statistic that
is most informative in deciding if one fit is better than another.



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