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

[Matt Siebecker]

Hello Bruce,
 
Thanks for your response.  By “second shell fits” I mean that the best values for the first shell
were fitted then fixed and then the fitting R-range was moved to the second
shell. Similarly to how Matt Newville
describes here: 
 
http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2009-April/008779.html
 
He describes this as an acceptable approach, although, others in
the thread disagree.
 
Essentially, my question is why
do the F-test and the RCS results not agree with each other?  The F-test indicates model 2 may not be a
statistical improvement over model 1 while the RCS values show that model 2 is
definitely an improvement over model 1.  If I consider a reduction in the RCS value of >2x as significant,
then I would pick model 2.
 
However, can I apply this logic when fitting the second shell with
the best parameters for the first shell fixed and the R-ranges over the second
shell?
 
Thank you again,
Matt S
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