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PARAFAC equation / 1 PowerPoint Presentation
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PARAFAC equation / 1

PARAFAC equation / 1

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PARAFAC equation / 1

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  1. PARAFAC equation / 1 • PARAFAC is an N -linear model for an N-way array • For an array X, it is defined as • denotes the array elements • are the model parameters • F is the number of fitted components • denotes the residuals • The model parameters are typically grouped in N loading matrices

  2. Sulfonamide 0.16 Interferent 0.12 Intensity [A.U.] 0.08 0.04 0 25 50 75 100 Scan [ i - i ] 0 0.2 Sulfonamide Interferent 0.15 0.1 Intensity [A.U.] 0.05 0 240 280 320 360 400 440 l [nm] HPLC-DAD • HPLC combined with Diode Array Detection • Light absorbed at i-th wavelength • Light absorbed at j-th time and i-th wavelength • For F compounds and K samples

  3. I3 I3 I2 I1 I1 I1 I1 Matricisation / 1 • Matricisation is an operation that associates a matrix to a multi-way array • The number of possible matricisations increases with the array’s order • Notation: • Some matricisations are faster than others • A shiftdim operation can be implemented more rapidly using appropriate matricisations

  4. I3 I1 I2 I1 I2 I2 I2 I2 Matricisation / 1 • Matricisation is an operation that associates a matrix to a multi-way array • The number of possible matricisations increases with the array’s order • Notation: • Some matricisations are faster than others • A shiftdim operation can be implemented more rapidly using appropriate matricisations

  5. Vectorisation • The vec operator transforms a matrix in a vector • In combination with matricisation one can define the vectorisation operation for N-way arrays • The result of the vectorisation depends only the order of the modes in resulting from matricisation

  6. Matricisation / 2 • The order of the modes is often taken as a convention • Row/Column modes in increasing/decreasing order • Row/Column modes in cyclycal order • Subscripts: –n, –nn', or –{n,n',} indicate the modes that are removed: • Subscripts n, nn', or {n,n',} for a matricised array indicate the modes in the rows

  7. Commutation matrices • For an n p matrix X, the commutation matrix Knp performs the operation: • For an I1 … INarray , the N-way commutation matrices Mn and Mnn'perform the operations: • Commutation matrices can be used to shift through matricisations • With cyclic modes notation shiftdim does not require commutation matrices

  8. The Khatri-Rao product For A and B with the same number of columns The column-wise Khatri-Rao product  performs the operation

  9. PARAFAC equation / 2 • The matrix equation for PARAFAC is • The vector representation of the PARAFAC model array is: • The notation is simplified using the letter Zfor the Khatri-Rao products • Different matricisations/vectorisation corresponds to permutations of factors in the Khatri-Rao product:

  10. Fitting the PARAFAC model • Fitting the PARAFAC model in the least squares sense corresponds to solving the nonlinear problem: • A weighted least squares fitting criterion takes the form where Dw is a (positive semidefinite) diagonal matrix holding the elements of w = vecW1 • If a the residuals variance/covariance S matrix is known:

  11. Algorithms for PARAFAC Many algorithms have been proposed to fitting PARAFAC models: • Alternating Least Squares (1970) • Gauss-Newton (1982) • Preconditioned Conjugate Gradients (1995/1999) • Levenberg-Marquardt (1997) • Direct Trilinear Decomposition (1990) • Alternating Trilinear Decomposition (1998) • Alternating Slice-wise Decomposition (2000) • Self-Weighted Alternating TriLinear Decomposition (2000) • Pseudo-Alternating Least Squares (2001) • PARAFAC with Penalty Diagonalization Error (2001)

  12. Alternating Least Squares • ALS breaks down the nonlinear problem in linear ones, which are solved iteratively Initial values for N-1 loading matrices must be provided • The properties of the Moore-Penrose inverse and those of the Khatri-Rao product are used to reduce the computational load • Convergence is checked at each step using (among others) the relative fit decrease

  13. PARAFAC-ALS Revisited • Using matricisations, rearrangements can be avoided or largely reduced • The computation load can be reduced by • a factor I1F–1 for a 3-way array for modes 2 and 3. • a factor InIn+1F–1for 4-way arrays and higher every two treated modes (n and n + 1) • Operating column-wise the number of operations is reduced by a factor F • The loss function can be calculated without explicitly calculating the residuals

  14. Line search extrapolation • Line search extrapolation is used to accelerate convergence in ALS • An analytical solution to the exact line search problem for PARAFAC • The optimal step length is found as the real root of a polynomial of degree 2N. • The cost for computing the polynomial coefficients directly is • A great reduction in the number of iterations is obtained with simple and exact line search

  15. Line search extrapolation • The computation time for is higher with exact line search • The problem seems to be in the search direction and not only the higher computation load per iteration • The algorithm in its fastest implementation seems to suffer from numerical instability • Several possibilities may prove beneficial • Perform line search only when the updates become highly collinear • Set the direction of search as the combination of several consecutive updates

  16. Self-Weighted Alternating TriLinear Decomposition • Does not find the least squares solution, but minimises at each step a modified loss function • Not straightforwardly extendible to higher orders • Requires full column rank for all loading matrices • The scaling convention affects the convergence • Similar cost as PARAFAC-ALS

  17. SWATLD • SWATLD fitting criterion and convergence properties are not well characterised • SWATLD yields biased loadings, which affects predictions • SWATLD yields solutions with higher core consistency • The results suggest that introducing such bias may be beneficial • Naïve solutions (PARAFAC-PDE) lead to unstable algorithms

  18. Levenberg-Marquardt • Based on a local linearisation of the vectorised residuals (r) in the neighbourhood of the interim solution • J is the Jacobian matrix of the vector of the residuals: and in matrix form it is expressed as • An update to the solution is found by solving the problem

  19. Jacobian: J • J is very sparse, with density • J is rank deficient because of the scaling indeterminacy • J is very tall and thin and cannot be stored as full apart from small problems • Sparse QR methods are unfeasible in most cases • The problem is solved using with system of normal equations:

  20. JTJ and JTDwJ • Both can be calculated without forming J • WLS case is much more expensive because of the calculation of U and V • Time expense can be reduced using property e. and c. of the Khatri-Rao product • Filling the sparse J and compute JTJ explicitly is faster for some WLS problems

  21. Gradient: JTr • Residuals are not necessary for LS fitting criterion • Faster routines based on the chain rule for matrix functions can be obtained using property e. of KR product • Complexity is identical to an ALS step

  22. Time consumption

  23. PARAFAC-LM • The size of the problem is • The cost per iteration is in the order of • The method is too expensive for large problems

  24. A comparison of algorithms • SWATLD and PARAFAC-LM are more resistant to mild model overfactoring • SWATLD did not yield 2 factor degeneracies in simulated sets. • PARAFAC-LM performs better for ill-conditioned problems • PARAFAC-LM is unfeasible for larger problems • SWATLD is faster than ALS and LM and relatively robust with respect to high collinearity • PARAFAC-LM is preferable for higher order arrays and if rank is relatively small

  25. Compression • The array is projected on some truncated bases • SVD based compressions • Tucker based compressions • Prior knowledge (CANDELINC, PARAFAC-IV) • The array is compressed to FN • Not compatible with non-negativity constraints

  26. QR compression / preconditioning • Calculate a QR decomposition of each loading matrix An=QnUn with Un upper triangular • Premultiply x withQN    Q1 • J becomes extremely sparse • Many data elements can be skipped • QR compression is lossless, but the compression rate is lower than for Tucker based compression

  27. Missing values • Several patterns of missing values • Randomly Missing Values (RMV) • Randomly Missing Spectra (RMS) • Systematically Missing Spectra (SMS) • Two approaches: • Weighted Least Squares (INDAFAC) • Single Imputation (ALS with Expectation Maximisation) • The conditioning of the problem is influenced by the • fraction of missing values • pattern of the missing values in the array

  28. Missing values: some results • Different patterns of missing values yield different artefacts • The quality of the predictions depends on the pattern • With RMV good predictions are possible also with 70% m.v. • Quality of the loadings varies with the asymmetry of the pattern • SMS pattern ”interacts” with multilinearity • INDAFAC grows faster with the % of missing values (RMV/SMS) • INDAFAC is faster than ALS for the SMS pattern

  29. Final remarks • There appears to be no method superior to any other in all conditions • There is great need for numerical insight in the algorithms. Faster algorithms may entail numerical instability • Several properties of the column-wise Khatri-Rao product can be used to reduce the computation load • Numerous methods have not been investigated yet