[Ifeffit] statistical parameters

Stefano Ciurli stefano.ciurli at unibo.it
Thu Dec 15 03:06:02 CST 2005

Hi Shelly,

>[Kelly, Shelly D.] Well, In my opinion that depends on where you are
>publishing the data.  If you are publishing the data in PRB then you
>should use the XX.X number.  If you are publishing in Science Magazine
>then you should use the rounded number.

clear as water :-)

>  If the answer depends on
>weather or not you round the number then I'd be suspicious.

this is not my case. I will use the XX.X number in the table. I will 
submit the paper to a biophysics type journal. I have now the 
arguments to have a dialogue with an hypothetical referee.

>Often we
>round the number so that we don't have to explain why we have a
>non-integer numbers.


>You need ~ a factor of 2 for there to be a statistically significant
>difference in the reduced chi squared.  So 33.5, 30, and 40 are all
>about the same number.  Therefore I would round them all off to the
>nearest whole number that works.   I like to show changes in the reduced
>chi square rather than the absolute value.  Instead of showing a
>reduced-chi-square of 100 and 350, I show them as 1 and 3.5. Then you
>can say that 3.5 is much bigger than 1 so the model that gave 1 is the
>best.  It is the relative change in the reduced chi square that matters.

got it

>We get the absolute value wrong for a number of reasons. By showing the
>change you can side step all that explanation.


>  > I know, but since I am reporting in a paper we are writing a table in
>>  which we show chi^2, reduced chi^2, Nidp and Npar (and therefore the
>>  degress of freedom) I was trying to be tidy and rigorous. But I am
>  > facing numbers which I cannot make much sense of.
>[Kelly, Shelly D.] That seems to be a problem.  Are you processing the
>data with more than one k-weight in IFEFFIT?

no, we use only a k^3 weight

I'll go read now Matt's mail...


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

"Fatti non foste a viver come bruti,
ma per seguir virtute e canoscenza"
Dante Alighieri - Inferno - Canto XXVI

"Ihr seid bestimmt, nicht Tieren gleich zu leben,
Nein, Tugend zu erringen und Erkenntnis"
"Ye were not form'd to live the life of brutes,
But virtue to pursue and knowledge high"

More information about the Ifeffit mailing list