[Ifeffit] R-factor and reduced chi square

[Wayne Lukens]

Hi Owen,

Whether one fit is statistically different from another is addressed by
Hamilton (W.C. Hamilton, Acta Cryst. (1965) V. 18, 502-510).  This 
tests is
directly applicable to EXAFS data, but there are a couple of caveats.
First, the number of data used to determine the number of degrees of
freedom is not the number of data points but the number of independent
points (Artemis will tell you this).  Second, if I recall correctly, the
R-factor used by ifeffit is the square of the crystallographic R-factor,
so you need to take the square root of the R-factor given by ifeffit
to use Hamilton's test.

Having said all that, it looks to me like neither fit is significantly 
better,
but I would need to know the number of parameters and number of
independent points to be sure.

Sincerely,

Wayne Lukens

On Oct 4, 2005, at 11:07 AM, OWEN N LI wrote:

> Hi,
>    I understand R-factor and reduced chi square are statistical ways to
> see if the model is reasonable (R-factor) and if one model is better
> than the others (reduced chi square). But how much should I read into 
> it?
>
>     I have model A and B, I believe A is more theoretical sound than B.
> but when I fit them using IFEFFIT, the R-factor and reduced chi square
> of B is slightly better than A (B: 0.03, 28; A: 0.04, 33). What does
> that tell me? Is my assumption incorrect? Or are they more or less the 
> same?
>
>     Owen
>
>
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