[Ifeffit] question about S02 (passive electron reduction factor)

[Kelly, Shelly D.]

Yu-Chuan, Matt and all,

> >> Yu-Chuan wrote:
> >>
> >> At the beginning of the fitting, the S02(amp) are far away 
> from 0.9 
> >> (just about 0.3). It's much different than Bruce's and Scott's 
> >> examples. [...] Also, John have said that the S02 value should be 
> >> around 0.9. That's why I am wondering if there is anything 
> wrong with 
> >> my data processing? Does anyone happen to have this kind 
> of problem?
> > 
> >
> > There are several possibilities. One is sample prep. [...] [...] 
> > Another is normalization.[...] Finally, look at the correlation 
> > between S02 and sigma2...if it is very high (say 0.95 or 
> above) it may 
> > simply be coming up with low estimates for each.
> 
> Yu-Chuan didn't give details of sample prep or data 
> collection, and I agree with Scott that these are most likely 
> cause of low S02 factors (Scott mentioned problems with 
> transmission measurements, and Grant mentioned how 
> self-absorption can cause similarly low S02 factors for 
> fluorescence measurements).  I also agree with Scott that 
> poor normalization will affect S02, though S02~0.3 would 
> indicate an edge step off by a factor of ~3, which is a 
> pretty big error for normalization.  I doubt that is the culprit.
> 
> But: the correlation between S02 and sigma2 is NOT the cause 
> of a value for S02 being low by a factor of 3.  A high 
> correlation means that a reasonably good fit could also be 
> found by changing both S02 and sigma2 away from their 
> best-fit value.  This is not at all the same as saying that 
> either parameter has the wrong best-fit value. In fact, the 
> correlation between pair of variables says nothing about the 
> trustworthiness of the best-fit values, only how much the 
> uncertainties in the best-fit values depend on other variables.  
> And, just to be clear, the reported uncertainties already 
> take these correlation into account.
> 
> Yu-Chuan didn't actually say what the reported uncertainties 
> in S02 were, but it seems that it was small enough so that 
> S02~=0.3 could be distinguished from S02~=0.9.
> 
> --Matt

The degeneracy of the paths included in the fit were not mentioned, and
could easily cause the S02 value to be off by a factor of 3.
Especially, if you forget that feff has already taken the degeneracy of
the path into account, and then you set some N-value to 2 or 3 or 4. I
have seen others make this mistake before...

Shelly