[Ifeffit] statistical parameters

Stefano Ciurli stefano.ciurli at unibo.it
Wed Dec 14 10:21:04 CST 2005


same old question that emerges again:

What formula is used by IFEFFIT to calculate the number of 
independent points? Does it use the formula 2DkDR/pi? or 2DkDR/pi +1? 
or 2DkDR/pi +2? And how does it approximate the resulting number to 

Next: as far as I understand, the conversion from chi^2 to 
reduced-chi^2 involves dividing the first by the number of degrees of 
freedom. The latter is obtained by subtracting the number or floating 
parameters from the number of independent point right?. What if one 
fixes the value of one parameter, let's say S0^2 to unity? Does the 
number of parameters includes S0^2 in that case? We consider that it 
does not.

Here is an example. We find with IFEFFIT (we use Dk= 10 (from 2 to 
12), DR=5 (from 1 to 6).

chi^2 = 87.1
reduced chi^2 = 3.7
parameters: E0, three distances and three sigma^2
we set S0^2 fixed to 1
R-factor: 1.15%

The ratio between chi^2 and reduced chi^2 is 23.54, which is not an 
integer number as we would expect. Plus, this number does not 
correspond to Ninp -Nvar (31.83 - 5 = 26.83)

What are we missing? Sorry if the question sounds silly to most of you.


Stefano Ciurli
Professor of Chemistry
Laboratory of Bioinorganic Chemistry
Department of Agro-Environmental Science and Technology
University of Bologna
Viale Giuseppe Fanin, 40
I-40127 Bologna
Phone:	+39-051-209-6204
Fax:	+39-051-209-6203

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