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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