[Ifeffit] Question about PCA

Joselaine Cáceres gonzalez joselainecaceres at gmail.com
Tue Apr 23 14:34:07 CDT 2019

Dear all,

I performed PCA in a set of 11 XANES spectra and I was trying to use some
of the functions and tests developed by Malinowski to extract the number of
primary components.
I found differences between eigenvalues calculated whit a matrix calculator
and those calculated by ATHENA, but they are equivalent in some way since
they explain the same amount of variance and when I calculate the IND
function or  Malinowski F-test I reach the same conclusions.
To  calculate the eigenvalues with the matrix calculator I first normalized
my data matrix (step height normalization) with ATHENA, then I export the
data to an Excel spreadsheet where I centered them:
z=(value-mean)/standard_dev. Then I used SVD and Eigenvalue decomposition
to find the eigenvalues and those obtained with SVD explain exactly the
same amount of variance than those found with ATHENA but they are
different. In all the calculation I did, I used the same amount of points
161, points in each spectrum in ATHENA and in the matrix calculator,
exactly the same energy interval.
I´d like to know if I am missing some step in the data pre-treatment that
is prevented me to find the same eigenvalues obtained by ATHENA but also
I´d like to know if I can straight forward use the values obtained with
ATHENA to evaluate the functions proposed by Malinowski since these
equations might be different depending if calculations are made using
"covariance about the origin" or "correlation about the origin" and I am
not sure what is the case.

Thanks in advance!
Best regards
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