Hi Matt, thank you for your answer!. The references I have about Malinowski´s work and some applications are: Malinowski, E.R., *Theory of error in factor analysis.* Analytical Chemistry, 1977. *49*(4): p. 606-612. Malinowski, E.R., *Theory of the distribution of error eigenvalues resulting from principal component analysis with applications to spectroscopic data.* Journal of Chemometrics, 1987. *1*(1): p. 33-40. Malinowski, E.R., *Statistical F-tests for abstract factor analysis and target testing.* Journal of Chemometrics, 1989. *3*(1): p. 49-60. Malinowski, E.R., *Adaptation of the Vogt–Mizaikoff F-test to determine the number of principal factors responsible for a data matrix and comparison with other popular methods.* Journal of Chemometrics, 2004. *18*(9): p. 387-392. McCue, M. and E.R. Malinowski, *Target Factor Analysis of the Ultraviolet Spectra of Unresolved Liquid Chromatographic Fractions.* Applied Spectroscopy, 1983. *37*(5): p. 463-469. Beauchemin, S., D. Hesterberg, and M. Beauchemin, *Principal Component Analysis Approach for Modeling Sulfur K-XANES Spectra of Humic Acids.* Soils Science Society of America Journal, 2002. *66*: p. 83-91. Wasserman, S.R., et al., *EXAFS and principal component analysis: a new shell game.* Journal of Synchrotron Radiation, 1999. *6*: p. 284-286. I think I don't understand what you mean by "eigenvalues explain exactly the amount of variance ... but they are different". Can you clarify? Giving an actual example might help. Here are results obtained by ATHENA, the factional variance explained by each eigenvalue is calulated by dividing the eigenvalue between the sum of them all, right?: # Eignevalues Variance Cumulative variance 1 8,864394 0,80585 0,805854 2 1,227578 0,11160 0,917452 3 0,708334 0,06439 0,981846 4 0,129478 0,01177 0,993617 5 0,045127 0,00410 0,997719 6 0,012229 0,00111 0,998831 7 0,009464 0,00086 0,999692 8 0,001489 0,00014 0,999827 9 0,000989 0,00009 0,999917 10 0,000617 0,00006 0,999973 11 0,000298 0,00003 1 Here are results obtained with matrix calculator for the same data: Eigenvalue Explained Variance Cumulative variance 1418,3057 0,80586 0,80586 196,4118 0,11160 0,917453 113,3331 0,06439 0,981846 20,7162 0,01177 0,993617 7,2205 0,00410 0,997719 1,9566 0,00111 0,998831 1,5144 0,00086 0,999692 0,2381 0,00014 0,999827 0,1583 0,00009 0,999917 0,0987 0,00006 0,999973 0,0477 0,00003 1,000000 The eigenvalues are used then to evaluate the function IND and F test, and depending on the values of eigenvalues, function IND reach a minimum value when the set of primary components are separated from the secondary ones that just explained experimental errors (in the equations lambda are the eigenvalues, r the numbers of rows, c columns, n the number of primary components): [image: image.png] The results obtained with the two sets of eigenvalues are diferent but they reach the minimum in the same n. The F test also gives me similar levels of significance for the two sets, but I do not undestand why I´m not hable to find the same eigenvalues that ATHENA does. By the way, I tested the possibility you told to not divide the data by the standard deviation and still couldn´t find the same eigenvalues. Best regards. Joselaine El mar., 23 abr. 2019 a las 23:15, Matt Newville (< newville@cars.uchicago.edu>) escribió:
Hi Joselaine,
On Tue, Apr 23, 2019 at 2:35 PM Joselaine Cáceres gonzalez < joselainecaceres@gmail.com> wrote:
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 am not familiar with the details of those tests. Do you have a reference for them?
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.
I believe that Athena does not do this centering. I would imagine that dividing by the standard deviation could skew the data.
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.
I think I don't understand what you mean by "eigenvalues explain exactly the amount of variance ... but they are different". Can you clarify? Giving an actual example might help.
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.
I don't know the answer to any of those questions.
I can say that when I compare PCA with Athena and with Larch (using scikit-learn's PCA), I do get very similar looking eigenvectors for the first few eigen components. To be clear, scikit-learn's PCA does first subtract out the mean (but does not divide by the standard deviation), whereas Athena identifies this as the 1st component. So there is a potential "off by 1" counting issue, but that is easily worked out.
The scalar values I get with scikit-learn's PCA are different from what Athena reports. scikit-learn PCA returns both the eigenvalues (the explained variance) and the explained variance ratio -- weights that will add to 1 . I generally find the latter to be more useful, but maybe I'm not understand what the eigenvales can be used.
Hope that helps. FWIW, I'm trying to learn all this stuff better too. Perhaps you can give some insight?
--Matt _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit
