# Use of the Gibbs sampler to invert large, possibly sparse, positive definite matrices

## Abstract

A problem that is frequently encountered in statistics is that of computing some of the elements (e.g., the diagonal elements) of the inverse of a large, possibly sparse, positive definite matrix. Let W = {wij} represent an m × m positive definite matrix, let V = {vij} = W-1, and let x = {xj} represent an m × 1 random vector whose distribution is multivariate normal with null mean vector and variance-covariance matrix V. The Gibbs sampler can be used to generate a sequence of m × 1 vectors x(1),x(2),... such that, for sufficiently large values of k, x(k) can be regarded as a sample value of x. Letting x(k)l,...,x(k)m represent the elements of x(k), x(k)i is a draw from a univariate normal distribution with mean -w-1ii (∑i-1j=1 wijx(k)j + ∑mj=i+1 wijx(k-1)j) and variance w-1ii. The sample values of x can be used to obtain a Monte Carlo estimate of the expectation E(xixj) and hence [since vij = E(xixj)] of vij. A possibly more efficient alternative is to use the sample values to evaluate the outer expectation in the expression E[E(xixj|xT)], where xT is a subvector of x that excludes Xi and/or xj. Sparsity (or various other kinds of structure) in W can be used to (computational) advantage in generating the draws x(1),x(2),.... Numerical results indicate that if W is well-conditioned, then the statistical dependence between the draws is relatively small, reasonably accurate Monte Carlo estimates can be obtained from a relatively small number of draws, and the conditioning of xixj on xT leads to significant improvements in accuracy. © 1999 Elsevier Science Inc. All rights reserved.