[Ifeffit] Query regarding error bars

Bruce Ravel bravel at bnl.gov
Fri Jan 9 09:27:05 CST 2009

On Friday 09 January 2009 09:12:16 am Bindu R. wrote:
> Could any tell me in a simple language about the error bars returned by the
> EXAFS fitting program? 
> what do they exactly represent?
> How is it determined?
> How is the number of iterations decided.
> In addition to R-factor what are the other parameters which determines a
> good fit. For a R-factor ~0.001, if the value of chi2~10,000 and reduced
> chi2 ~ 500, can one consider the fit to be good even if one gets a good
> match to the experimental spectra.

I believe that most of these questions are answered at

The number of iterations is determined from the data.  That is, when the fit 
converges to within some tolerance, the fit stops and the error bars are 
evaluated.  There is a hard-wired default for that tolerance and long 
experience suggests it is reasonable.  There is also a hard-wired upper limit 
to the number of iterations, but a decent fit never reaches that number.

The error bars are the diagonal elements of the covariance matrix evaluated 
during the fit.  Because uncertainty is so hard to properly evaluate (as 
discussed in the links in the FAQ), the error bars are rescaled under the 
assumption that the fit performed was, in fact, a good fit.  Thus the error 
bars are a reasonable (although probably conservative) estimate of 
measurement uncertainty if you believe that the fit is indeed a good fit.  
Again, see the FAQ for some hints about how to decide if a a fit is a good 

If you have additional specific questions, let us know so we can answer them 
here and update the FAQ as needed.



 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

More information about the Ifeffit mailing list