[Ifeffit] Re: Independent data poins in Artemis

["Sang-Wook Han"]

Hi! Bruce,

I have checked the Matt's note.
Although I understand the problem, it is still not clear for me.
If we have only one data point in k-space, it is a delta function. For 2DrDk/pi, it should be zero.
When we do Fourier transformation of the one data point, we still have data points in real and imaginary r-space. That is not zero.

Han.





----- Original Message -----
From: "Bruce Ravel" <bravel at anl.gov>,
To: "Sang-Wook Han" <swhan at chonbuk.ac.kr>,"XAFS Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>,
Date: 2007-01-04 23:53:20
Subject: Re: Independent data poins in Artemis

On Thursday 04 January 2007 03:06, Sang-Wook Han wrote:
> Hi! Bruce
>
> I have a question of the independent data points.
> I calculate the indenpent data points using Stern's idea,
> N = 2(Kmax - Kmin)*(Rmax-Rmin)/3.14 + 2.
> As you can see the attached file, the result of the Artemis is different
> from this method. How do you determine the independent data points?

Hi Han,

I am taking the liberty of CCing my response to the Ifeffit mailing
list since this is a very common question and quite approriate for
discussion in that forum.

The issue of enumerating independent points is one that has been
discussed at some length already on the mailing list.  Here are some
links.  In this one, Matt summary is quite consistent with my view on
this issue:

  http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2005-December/002222.html

Here Scott makes reference to a paper applying Baysing approaches to
EXAFS data analysis and to the fact that the EXAFS measurement is not,
in fact, an ideally packed signal.

  http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2005-December/002221.html

It seems to me that if your analysis requires the "+2", then you are
almost certainly placing excessive demand on your data.  Almost
certainly, you are using parmaters that are highly correlated and are
seeing quite large error bars.  If that is the case, you will need to
think about how to use constraints to simplify your fitting problem.
See question 3 at http://cars9.uchicago.edu/iffwiki/FAQ/FeffitModeling.


Regards,
B

P.S. It would be very helpful if someone could make am entry on that
same FAQ page discussing the issue of enumerating Nidp.


-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
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 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
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