Re: [Ifeffit] Standard deviation of R value

As an additional comment, it is a murky issue whether the reported thing together with R is the standard deviation. It is not easy to find it in the literature but there are some papers dating back to the 1990s that state that the uncertainties reported in the EXAFS fitting results correspond to the 95% confidence level which means 2 standard deviations for an approximately normal data set. I don't think it is correct. Regardless whether the distribution of some or all fitting variables are treated by the Levenburg-Marquartd method as normal or not, the reported error is really not a standard deviation but a square root of the element in a covariance matrix. What is the relationship of that and the standard deviation of the fitting variable is one of the few remaining unsolved mysteries in EXAFS analysis. Anatoly On Fri, Jan 11, 2019 at 11:05 AM Matt Newville wrote:

Hi Erin,

On Fri, Jan 11, 2019 at 8:19 AM Erin Finley wrote:

...

Hello, I see in many publications the R value reported with a standard deviation. However, when I look at my resulting data from a fit I don't see one listed. I am still new to EXAFS research and so I am wondering where I can get this information. I am assuming I need to calculate it from another parameter I am just not sure which one.

The uncertainties in R are typically in the fit report. But also, we typically refine a "delR" parameter that is R - Reff, where Reff is the distance (half path-lengh) for the scattering path. So, if you don't see an uncertainty reported for R but do see an uncertainty for delR (or whatever parameter name you used), use that. The uncertainty in R will (normally) be the uncertainty in delR.

If I'm misunderstanding the question, pleas give more details about what you are doing.

--Matt

Thanks for your help!

...

Cheers,

-- Erin Finley PhD Candidate Brgoch Research Group Department of Chemistry University of Houston _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit

-- --Matt Newville <newville at cars.uchicago.edu> 630-252-0431 _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit