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

[Matt Newville]

Hi Bruce,

On Thu, Oct 2, 2008 at 4:12 PM, Bruce Ravel <bravel at bnl.gov> wrote:
> 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.

Does it?  I thought it used data_uncertainty=1 unless the uncertainty
was explicitly specified.  Does Athena use a value determined from the
sigma when merging data, or is something?   Should I read the Users
Guide more closely?

--Matt