[Ifeffit] XANES fitting

[Ritimukta Sarangi]

Hi Matt,
Thank you. I am looking forward to comments from members from the forum. Yes, I have found some of Corwin's references. Perhaps it might help current and future EXAFS folks to have a page on the Ifeffit website discussing the causes of such systematic and random errors that you point out and listing some of these references?
Best,
-Riti


On Oct 12, 2011, at 11:35 AM, Matt Newville wrote:

> Hi Riti,
> 
> I hope it's OK that I'm sending this to the ifeffit mailing list, as
> there might be other interesting perspectives on your questions.
> 
> On Tue, Oct 11, 2011 at 3:47 PM, Ritimukta Sarangi <ritis at stanford.edu> wrote:
>> Hi Matt,
>> This is Riti again. This time I have a question about EXAFS, which you are an expert on :-) I would like to get your view on
>> systematic errors in EXAFS. Say the data were collected on a standard EXAFS beamline like 7-3 at SSRL. The data have
>> been collected using fluorescence mode and a 30 element Ge detector. The data are modest, between k=2-15 A.
>> 
>> I used to quote in my papers that typical statistical and systematic errors lead to a total error in first shell distances of
>> +-0.02 A.  I know it can be more than that in some circumstances but then I tried to look at it a little more closely to
>> separate out the systematic error from all sources (instrument, measurement, detector, Analysis), but I could not find a
>> good reference. I understand that this is a difficult number to estimate. I was hoping that you could shed some light on it.
>> Thanks!
>> -Riti
>> 
> 
> I'm not sure of a good general reference for that either, though
> perhaps some of the papers from Corwin Booth are a good place to
> start?   Quoting +/- 0.02 Ang seems like a reasonable estimate for a
> typical value, but that's hardly rigorous.
> 
> My sense is that it's pretty challenging to separate "systematic" and
> "random" noise for most data sets.  Fluorescence data from a
> solid-state detector is probably the best-case for such a separation,
> because you have multiple detector elements or repeated scans,  and
> usually the individual detectors or scans really are dominated by
> statistical fluctuations.   If you add detector elements or scans
> together until the noise in the data (or variations in estimated
> parameters or estimated error bars)   stops getting better,
> systematic errors are starting to be noticeable. Eventually, as you
> increase the fluorescence counts, the fits will stop improving, which
> should be when systematic errors dominate.
> 
> --Matt