[Ifeffit] S02-Interpretation

[Kelly, Shelly D.]

Hi Matt

> > Now, if I have two samples and determined the S02 value according to
> > the way which Bruce has in his supplement to the FEFFIT course, 
> 
> How do you mean this?  Is this the 'plot three curves of sigma2 v.  
> S02 curves for three different k-weights' again?  I find this 
> approach puzzling and dangerous.  I do not understand why the slope
> (correlation) of sigma2 v. S02 should depend on k-weight in 
> any systematic way -- does anyone else know why it should??  
> What you want is the S02 and sigma2 that gives the lowest 
> chi-square, not where these lines cross.

I have found that usually there is a "larger than one would like"
correlation between the amplitude and phase terms in the EXAFS equation.
But the different terms have different k-dependencies.  Low k-weights
will give more weight to an accurate E0, whereas higher k-weights will
give a more accurate deltar.  Although the errors will be large.  

If you plot the dependence of these variables on the k-weight, or better
yet use all three k-weights in the fit, you will find that the best-fit
value is consistent with the one k-weight value but that the uncertainty
is lower because you are distinguishing between the two coorelated
variables by including the k-dependence in the fit.

Shelly