[Ifeffit] Ifeffit Digest, Vol 220, Issue 2

Danilo Oliveira de Souza danilo.olsouza at gmail.com
Tue Jun 15 04:03:20 CDT 2021


Hi Teresa and Matt,

Concerning the SPOIL function, some months ago I digged on that and I found
the first paper (I guess) on this topic, which is from Edmund R. Malinowski
in an Analytical Chimica Acta, 1978 ( 10.1016/S0003-2670(01)83099-3
<https://doi.org/10.1016/S0003-2670(01)83099-3> ).
Instead of reproducing this paper here (it is not just a couple of lines),
I let to you read the precise definition of such a function.
I have the copy on pdf of the paper if any of you is interested in it.

Cheers,
  Danilo



*Danilo OLIVEIRA DE SOUZA*

*Research Fellow @ ELETTRA Sincrotrone, XAFS beamline*

*    Strada Statale 14 - km 163,5 in AREA Science Park*
*    34149 Basovizza, Trieste ITALY*
    *http://www.elettra.trieste.it/elettra-beamlines/xafs.html
<http://www.elettra.trieste.it/elettra-beamlines/xafs.html>*

*   e-mail: danilo.oliveiradesouza at elettra.eu
<danilo.oliveiradesouza at elettra.eu> *

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Em qui., 10 de jun. de 2021 às 19:00, <
ifeffit-request at millenia.cars.aps.anl.gov> escreveu:

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>    1. Re: PCA Sixpack results compared to Athena (etc.) (Matt Newville)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 10 Jun 2021 07:08:19 -0500
> From: Matt Newville <newville at cars.uchicago.edu>
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] PCA Sixpack results compared to Athena (etc.)
> Message-ID:
>         <CA+7ESbq=
> J9fZ3Y0PPuyjShk+XNFOGjOTktX4m7oyu6PFfTwM5w at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hi Teresa,
>
> I'm sorry that I cannot give you a definitive answer.  I should admit that
> when adding PCA methods to Larch and XAS Viewer (which I invite you to try
> out), I tried to follow the scikit-learn approach but also to follow the
> Athena implementation.  I think I never tried to test against the results
> from SixPack.   FWIW, Larch only does (easily) PCA on normalized mu(E) or
> its derivative.  I suppose PCA on chi(k) could be added, but I'm a bit sort
> of skeptical of this.
>
> In fact, the code at
> https://github.com/xraypy/xraylarch/blob/master/larch/math/pca.py (and,
> just to be clear, having this both in Python and publicly available is
> motivated by having these conversations of "what does it do?") has a few
> different methods to train a PCA set: one directly from scikit-learn, one
> basically reproducing Demeter's PCA.pm (modulo slight differences in
> underlying math libraries, which should be insignificant), one that aims to
> use only non-negative components (not really worth in my opinion), and one
> that is sort of hand-coded and including the IND statistic.   I don't know
> what SixPack does.
>
> I cannot really explain why, but the default "readily exposed in Larch XAS
> Viewer" is to use the hand-coded version of `pca_train`.  In fact, they
> should be all more or less interchangeable.  I did some tests with these
> but that was now several years ago, but it might be worth trying that
> again.  If you're up for that, please do try.  If not and would like to
> send your project and an outline of what you get, I might be able to look
> at this too.
>
> For "target transformation", this is implemented as `pca_fit`: how well can
> a data set be explained by the first N components of a training model?
>
> For fit statistics: I have seen "SPOIL" used several places in the EXAFS
> literature but am afraid I do not actually know of a definition for this.
> If anyone can explain what these are, that would be helpful.   Larch can
> calculate the F1 and IND statistics that are more common in the PCA
> literature.  XAS Viewer exposes and automatically plots IND - it's a very
> useful way to select how many components are significant.
>
> I'm pretty sure that does not answer your actual question, but maybe it
> will be helpful.  If you or anyone else has suggestions for additions,
> improvements, or other optional methods or statistics for PCA and/or
> related methods, please let me know.
>
> --Matt
>
>
> On Tue, Jun 8, 2021 at 7:43 AM <t.zahoransky at mineralogie.uni-hannover.de>
> wrote:
>
> >
> > Dear XAS Community,
> >
> > I stumbled over the issue that a PCA on 20 EXAFS spectra (k?, k =
> > 2.0-11.0 ?-1) perfomed in Sixpack does not give the same results
> > (Eigenvalues, variance) as in Athena. However, when I use other
> > statistical programs (i.e., TIBCO Statistica or SPSS), I get the same
> > results as reported in Athena. I tested this with another EXAFS
> > dataset of over 30 samples and the problem persited.
> >
> > An old entry from 2017 in the ifeffit mailing list ("[Ifeffit]
> > Calculation of SPOIL value for the reconstruction of standard
> > spectra"), told me that as of Sixpack version 1.4 on, a new/different
> > PCA algorithm from the scikit-learn Python package is used.
>
>
> > So I downloaded older versions of Sixpack (i.e. 1.3) and used "Use Old
> > PCA"-selection in the "Rotation" menu bar, which actually gave
> > different results. However, they are still different from the
> > Athena/Statistical program results.
> >
> > My question is: What is behind this? Is there some sort of
> > normalization or axis rotation, that leads to the different values? Is
> > there any way to change this so that the results are comparable to
> > other programs?
> >
> > As I need to use the Target Transform option after PCA, which is not
> > yet possible in Athena, I am at a loss as to how to deal with these
> > different results and where they come from.
> >
> >
> > Thank you very much for your help,
> >
> > Teresa
> >
> >
> > --
> > Teresa Zahoransky
> >
> > Soil Mineralogy
> >
> > Gottfried Wilhelm Leibniz Universit?t Hannover
> >
> > Institute of Mineralogy
> >
> > Callinstr. 3, Room 325
> >
> > D-30167 Hannover, Germany
> >
> >
> >
> > Phone: +49 (0)511 762-8058
> >
> > Email: t.zahoransky at mineralogie.uni-hannover.de
> >
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>
>
> --
> --Matt Newville <newville at cars.uchicago.edu> 630-327-7411
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