[Ifeffit] Fitting parameters corrleations between variables

Matt Newville newville at cars.uchicago.edu
Mon Nov 23 22:48:50 CST 2015


On Mon, Nov 23, 2015 at 8:59 PM, pushkar shejwalkar <
pshejwalkar2004 at gmail.com> wrote:

> Dear All,
>     I am trying to fit in Pd compound and the fit that I obtained looks
> kinda good. I used Pd metal and Pd(OAc)2 shells for fitting. These
> standards were collected and fitted simultaneously and I got the SO2 value
> there as 0.67 with Pd foil coordination number as 12 which is correct when
> considered Pd foil CN. Next while fitting the actual sample (unknown) I
> used the formula in SO2 column as amp*N1 and put amp as 0.67 and N as 1. I
> kept N1 as variable (Guess). same I did for the second shell of Pd-O with
> N2. amp is kept on SET value of 0.67. After fitting I received the N1 and
> N2 value as 5.6 (0.5) and 1.6 (0.7) (roughly). however, when I checked the
> correlations between the variables I got the values as given in the log
> file attached. I saw there is some correlation between bkg01_01 and N2 and
> many such correlations between bkg values. I have so far never seen such
> correlations. What are the implications of these correlations? does this
> means that my fitting assumptions are incorrect. does correlations values
> in negative indicates something?

Correlation between two variables A and B means that if you changed one
variable (A) away from its best-fit value, you could improve that fit by
changing the other variable (B).  In fact the correlation tells you by how
much, and (from the sign) in what direction that change would be.

A correlation of 0 means that if one changed A, B would not need to change
at all. A correlation of 1 would mean that a change in A by 1 standard
error (the reported error), B would have to change by 1 standard error.
And, again, the sign gives the direction of that change.

Having correlated values does not mean that your assumptions are incorrect.

Thank you very much for the input. By the way my RBKG is 1.2 in athena and
> I used this same file for fitting purpose and on artemis I used rmin 1.2
> and rmax 3.4. All these information are in log file attached
> Any help in understanding this is appreciated.
> --
> Best Regards,
> Pushkar Shejwalkar.
> Post-doctoral -Researcher,
> Tokyo Engineering University,
> Tokyo-to
> Japan
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