mathematics Article Geometric Ergodicity of the Random Walk Metropolis with Position-Dependent Proposal Covariance Samuel Livingstone Department of Statistical Science, University College London, London WC1E 6BT, UK;
[email protected] Abstract: We consider a Metropolis–Hastings method with proposal N (x, hG(x)−1), where x is the current state, and study its ergodicity properties. We show that suitable choices of G(x) can change these ergodicity properties compared to the Random Walk Metropolis case N (x, hS), either for better or worse. We find that if the proposal variance is allowed to grow unboundedly in the tails of the distribution then geometric ergodicity can be established when the target distribution for the algorithm has tails that are heavier than exponential, in contrast to the Random Walk Metropolis case, but that the growth rate must be carefully controlled to prevent the rejection rate approaching unity. We also illustrate that a judicious choice of G(x) can result in a geometrically ergodic chain when probability concentrates on an ever narrower ridge in the tails, something that is again not true for the Random Walk Metropolis. Keywords: Monte Carlo; MCMC; Markov chains; computational statistics; bayesian inference 1. Introduction Markov chain Monte Carlo (MCMC) methods are techniques for estimating expecta- Citation: Livingstone, S. Geometric tions with respect to some probability measure p(·), which need not be normalised. This is Ergodicity of the Random Walk done by sampling a Markov chain which has limiting distribution p(·), and computing Metropolis with Position-Dependent empirical averages. A popular form of MCMC is the Metropolis–Hastings algorithm [1,2], Proposal Covariance.