Hi Lisa, In general the R-factor is good if it is less than a few percent. The value reported is 0.01 or 1% then the fit is satisfactory. Just as a note the R-factor is calculated over the entire data range given by rmin and rmax of the fit, so make sure that they are reasonable values. To compare two different models, use the reduced-chi-square (RCS) value. One standard deviation in the RCS value is SQRT(2/nu), where SQRT is square-root and nu is the number of independent points. If the first model has RCS1 and nu1 and the second model has RCS2 and nu2 then the second model is better than the first model if their difference (RCS1-RCS2) is greater than 2*SQRT((2*RCS1*RCS1)/nu1 + (2*RCS2*RCS2)/nu2)) which is twice the fluctuation of the difference. The number of independent points is equal to the number of data points minus the number of variables in the model. I do this kind of comparison in this paper: S.D. Kelly, K.M. Kemner, G.E. Fryxell, J. Liu, S.V. Mattigod, K.F. Ferris, "X-ray Absorption Fine-Structure Spectroscopy Study of the Interactions Between Contaminant Tetrahedral Anions and Self-Assembled Monolayers on Mesoporous Supports," The Journal of Physical Chemistry B 105 (27) 6337-6346, Aug 2001. The paper can be found here: http://www.mesg.anl.gov/sdkpublist.html. Cheers, Shelly --------------------------- Shelly Kelly Argonne National Laboratory Bldg 203, RM E113 9700 S. Cass Ave Argonne, IL 60439 Molecular Environmental Science Group www.mesg.anl.gov skelly@anl.gov phone: 630-252-7376
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit- bounces@millenia.cars.aps.anl.gov] On Behalf Of Lisa Giachini Sent: Friday, January 05, 2007 6:23 AM To: XAFS Analysis using Ifeffit; FEFF Users Cc: XAFS Analysis using Ifeffit Subject: [Ifeffit] R-factor uncertainty
Dear all,
I have a question about the R factor: how can I decide if the difference between the R factors of 2 fits is statistically significant, i.e, how can I calculate the uncertainty which has to be associated to the R factor?
B.R.,
Lisa
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