[Ifeffit] What confidence limit is implied in the error bars for least-squares fitting in Athena

[Bruce Ravel]

On Thursday 02 October 2008 16:49:27 Andrew wrote:
> I'm fitting data with the least-squares method in Athena and am not sure
> what the error bars correspond to. What I mean, are these 95% confidence
> limits, or is it some other statistical way for error analysis?  If Athena
> says the weight of one phase is 0.049 (0.003), what confidence are in the
> 0.003 error bar reported by Athena?

They are 1-sigma error bars, with the caveat that they assume that the
only source of noise is statistical noise.  Since an XAS experiment is
(almost) never dominated by statistical noise, it's generally a very
conservative (in the sense that it almost certainly understates the
confidence) 1-sigma.

The dominant source of error in a typical XAS experiment is often
something like sample inhomogeneity.  I am not really certain how to
quantify that sort of thing, so I don't actually know how to report a
better error bar.

B

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

 National Institute of Standards and Technology
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