[Ifeffit] N independent variables

[Matt Newville]

Jesus,


On Mon, May 23, 2016 at 2:07 PM, Jesús Eduardo Vega Castillo <
jevecas at gmail.com> wrote:

> Dear list,
>
> I made an EXAFS analysis in Artemis (in Demeter 0.9.22) using all the
> available variables. This is a fragment of the .log file I got:
>
> Independent points          : 17.7666016
> Number of variables         : 17
> Chi-square                  : 265.0947132
> Reduced chi-square          : 345.8050781
>
> R-factor                    : 0.0026486
>
> Number of data sets         : 1
> : k-range               = 2.942 - 11.043
> : dk                    = 1
> : k-window              = hanning
> : k-weight              = 1,2,3
> : R-range               = 1.115 - 3.5
> : dR                    = 0.0
> : R-window              = hanning
> : fitting space         = r
> : background function   = yes
> : phase correction      = no
> : background removal    = E0: 20002.215, Rbkg: 1.0, range:
> [2.25:17.4039986832903], clamps: 0/24, kw: 2
> : epsilon_k by k-weight = 3.189e-004
> : epsilon_r by k-weight = 2.339e-001
> : R-factor by k-weight  = 1 -> 0.00220,  2 -> 0.00224,  3 -> 0.00392
>
> The problem is that when I use the Nyquist criterion
>
> Nind=2*deltak*deltar/pi + 1
>
> for calculating the number of independent points the value I got is much
> lower and close to 13.
>
> I was not aware of this discrepancy and it caused a reviewer to think I
> made up the fit!
>
>
Sending the entire log file is always recommended.  Otherwise, you are
selectively editing what you are telling us, and expecting us to guess what
you haven't told us.  Sending the project file is often a good choice.

Fortunately, you included an important clue.  You have "background function
= yes", which means the fit will actually extend down to R=0, using your
Rmin as Rbkg in calculating how many variables are used for the spline.
With that included, the fit does contain approximately 18 independent
parameters, about 5 of which will be used for the background subtraction.

--Matt
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