[Ifeffit] PCA Sixpack results compared to Athena (etc.)

[t.zahoransky at mineralogie.uni-hannover.de]

Dear XAS Community,

I stumbled over the issue that a PCA on 20 EXAFS spectra (k², k =  
2.0-11.0 Å-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 Universität 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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