[Ifeffit] Debye factor and S02 correlation

Bruce Ravel ravel at phys.washington.edu
Tue Nov 5 13:24:41 CST 2002

 jjr> I don't understand Bruce's remark. Incorporating prior knowledge
 jjr> into statistics is completely proper according to modern
 jjr> statistical theory. If two quantities are correlated and one
 jjr> knows one, then naturally the other is better determined.

Well, leaving aside the question of whether one can call a Levenberg-
Marquardt fit "modern", I agree that one can set a value and call it
part of the model.  One does that all the time in any fitting problem.
But in that case you are asserting a value with no error bar.  There
is no situation where you can assert a value AND an error bar.

My concern was your statement that you somehow knew the value of the
error bar on S02.  If it floats, you have to use the error bar the
fitting algorithm reports.

But the most important thing in my comment and the most important
thing in the discussion on the topic of amplitude terms in exafs that
we have seen in the last week on this list is that, in a difficult
fitting problem (i.e. not copper), one rarely has reliable a priori
knowledge of the *product* of S02, coordination, and the systematic
uncertainties.  Thus one rarely has a reliable prior in the Baysian


 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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 Naval Research Laboratory                          phone: (1) 202 767 5947
 Washington DC 20375, USA                             fax: (1) 202 767 1697

 NRL Synchrotron Radiation Consortium (NRL-SRC)
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 National Synchrotron Light Source
 Brookhaven National Laboratory, Upton, NY 11973

 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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