[Ifeffit] Reduced chi square values versus F-tests for second shell fits

[Matt Siebecker]

Dear All,
 
Would it be recommended to compare reduced chi square (RCS) values
from the fit of the second shell only?  While
fitting the second shell only, the differences between RCS values of different models
can be greater than when including both the first and second shells in the fit.  For example, the RCS values of model 1
and model 2 for fits of the first and second shells versus just the second
shell are 45 and 18, and then 60 and 17, respectively. This to me indicates
that fitting the second shell separately from the first gave me a slightly
better description of the misfit in that R-range.  Regardless of the fitting range the RCS values
say model 2 improves the fit significantly.
 
However, if I use an F-test to compare models while fitting the
second shell only, I’ve found that the difference between the models is actually
less important than when including both the first and second shells in the fit.
For examples, the F-test values of model 1 versus model 2 for fits of the first
and second shell versus just the second shell are P=0.047 and then P=0.113,
respectively.  This is perhaps caused by
the small R-range and hence decreased Nind when fitting the second shell only.
 
However, it also indicates to me that the F-test says perhaps one
fitting model is not such an improvement on the fit as I thought it was based purely
on the decrease in RCS value.  I assume significant
decreases in RCS values to be about >2x, and P <0.05 for the F-test.
 
Should RCS values and F-tests for comparing fitting models only be
used when fitting two shells together or can they be used when just fitting the
second shell?
 
Kind regards,
Matt Siebecker
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