[Ifeffit] statistical parameters
Stefano Ciurli
stefano.ciurli at unibo.it
Wed Dec 14 10:21:04 CST 2005
Hello,
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
integer?
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
--
____________________________________________
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
Italy
Phone: +39-051-209-6204
Fax: +39-051-209-6203
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