[Ifeffit] minimize function
Bruce Ravel
ravel at phys.washington.edu
Tue Jan 28 16:48:19 CST 2003
> On Tue, 28 Jan 2003, "Kelly, Shelly D." <SKelly at anl.gov> said:
SDK> Hi, I would like to use the minimize function to fit a line to
SDK> some data points. I could not find documentation on the
SDK> r-square factor that is referred to as the confidence level for
SDK> the fit.
I was just browsing through `iff_minimize.f'. I am sure that Matt
will correct me if I am wrong, but it seems as though no R-factor is
evaluated by a call to minimize() in ifeffit. Chi-square is
essentially the sum of squares of the function that you minimize
(which, in your case, presumably, is the data minus the line). It and
a reduced chi-square (normalized by the number of points in the
minimization function minus the number of variables) are evaluated and
stored as program variables, but there appears to be no R-factor that
is stored.
I just did a minimize and that seems to be the case, despite what it
says in chapter 7 of the Ifeffit tutorial on Matt's website.
If you were to make an R-factor that is the real-array equivalent of
Ifeffit computes when fitting (imaginary) R-space EXAFS data, it would
be something like:
Npts [function(i) - data(i)]^2
R = sum ----------------------------
i=i data(i)^2
That would be a sort of positive-definate, percentage misfit and would
be trivial to compute using ifeffit's array manipulation.
HTH,
B
--
Bruce Ravel ----------------------------------- ravel at phys.washington.edu
Code 6134, Building 3, Room 222
Naval Research Laboratory phone: (1) 202 767 5947
Washington DC 20375, USA fax: (1) 202 767 1697
NRL Synchrotron Radiation Consortium (NRL-SRC)
Beamlines X11a, X11b, X23b, X24c, U4b
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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