MN_MLE {MatrixLDA}

Matrix-normal maximum likelihood estimator

Description

A function for fitting the J-class matrix-normal model using maximum likelihood. Uses the so-called “flip-flop” algorithm after initializing at U = I_r.

Usage

MN_MLE(X, class, max.iter = 1000, tol = 1e-06, quiet = TRUE)

Arguments

X

An r \times c \times N array of training set predictors.

class

N-vector of training set class labels; should be numeric from \left\{1,...,J\right\}.

max.iter

Maximum number of iterations for “flip-flop” algorithm.

tol

Convergence tolerance for the “flip flop” algorithm; based on decrease in negative log-likelihood.

quiet

Logical. Should the objective function value be printed at each update? Default is TRUE. Note that quiet=FALSE will increase computational time.

Value

Returns of list of class "MN", which contains the following elements:

Mean

\bar{X}; An r \times c \times C array of sample class means.

U

\hat{U}^{\rm MLE}; the r \times r estimated precision matrix for the row variables.

V

\hat{V}^{\rm MLE}; the c \times c estimated precision matrix for the column variables.

pi.list

\hat{\pi}; J-vector with marginal class probabilities from training set.

References

• Molstad, A. J., and Rothman, A. J. (2018). A penalized likelihood method for classification with matrix-valued predictors. Journal of Computational and Graphical Statistics.

Examples

## Generate realizations of matrix-normal random variables
## set sample size, dimensionality, number of classes, 
## and marginal class probabilities

N = 75
N.test = 150

N.total = N + N.test

r = 16
p = 16
C = 3

pi.list = rep(1/C, C)

## create class means
M.array = array(0, dim=c(r, p, C))
M.array[3:4, 3:4, 1] = 1
M.array[5:6, 5:6, 2] = .5
M.array[3:4, 3:4, 3] = -2
M.array[5:6, 5:6, 3] = -.5


## create covariance matrices U and V
Uinv = matrix(0, nrow=r, ncol=r)
for (i in 1:r) {
	for (j in 1:r) {
		Uinv[i,j] = .5^abs(i-j)
	}
}

eoU = eigen(Uinv)
Uinv.sqrt = tcrossprod(tcrossprod(eoU$vec, 
diag(eoU$val^(1/2))),eoU$vec)

Vinv = matrix(.5, nrow=p, ncol=p)
diag(Vinv) = 1 
eoV = eigen(Vinv)
Vinv.sqrt = tcrossprod(tcrossprod(eoV$vec, 
diag(eoV$val^(1/2))),eoV$vec)

## generate N.total realizations of matrix-variate normal data
set.seed(1)
X = array(0, dim=c(r,p,N.total))
	class = numeric(length=N.total)
	for(jj in 1:N.total){
		class[jj] = sample(1:C, 1, prob=pi.list)
		X[,,jj] = tcrossprod(crossprod(Uinv.sqrt, 
		matrix(rnorm(r*p), nrow=r)),
		Vinv.sqrt) + M.array[,,class[jj]]
	}

## fit matrix-normal model using maximum likelihood
out = MN_MLE(X = X, class = class)