[Ifeffit] Debye factor and S02 correlation

[Bruce Ravel]

A few comments on this S02 and Ei thing:

1.  I am not particularly encouraged by the fact that Matt was just
    barely able to co-refine S02 and Ei in a copper foil.  In many
    ways a copper foil is a very contrived exafs problem.  I would
    like to see a careful study by someone who understands the
    statistics well on a wide variety of well-ordered and disordered
    materials before I would suggest to anyone that they consider
    reporting both S02 and Ei in the literature.  I am sure that S02
    and Ei will always be highly correlated and I doubt that, for most
    problems, they will have reasonable best-fit values despite their
    large correlations.  Of course, I would be pleased to be proven
    wrong. 

2.  I don't quite agree with John's assessment of the error bars.  The
    two parameters are almost completely correlated.  I don't think it
    is reasonable to say that two parameters which are that highly
    correlated and which have values of the same order of magnitude
    should have error bars of different orders of magnitude.  I
    understand that we have other reasons to know what an appropriate
    value for S02 should be and so I understand why John wants to
    state the S02 can be 0.9+/-0.05, but from the vantage point of
    interpreting the statistics of the fit, you are NOT at liberty to
    assert an error bar.  This is particularly true for a real
    research problem with difficult sample prep or detector issues.
    The amplitude really may be something like 0.75 after the fit
    accommodates such systematic uncertainties.  What's more,
    correlations with the various sigma^2 parameters used in a fit to
    a disordered material may drive up the error bar on S02 to
    something like 0.1 or 0.2 or higher.

In short, I have to caution everyone reading this list about the
dangers of drawing conclusions about hard analysis problems from the
results of an easy one like a copper foil.


B    


-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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