[Ifeffit] R-factor and reduced chi square

Michel Schlegel michel.schlegel at cea.fr
Wed Oct 5 06:44:18 CDT 2005


Hello,

Out of curiosity (and maybe also slackness; I peered over Matt's papers,
including Iffefit reference manual, but no equation popped to my eyes):
can I have a brief refresher on how the R-factor is calculated in Ifeffit?
(a reference will do).

Thank you in advance,

best regards - Michel


>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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-- 
Michel Schlegel
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