A study of how scan orders influence Mixing time in Gibbs sampling.

This paper is interested in comparing the mixing rates of Gibbs sampling using either systematic scan or random updates. The basic contributions are two: First, in Section 2, a set of cases where 1) systematic scan is polynomially faster than random updates. Together with a previously known case where it can be slower this contradicts a conjecture that the speeds of systematic and random updates are similar. Secondly, (In Theorem 1) a set of mild conditions under which the mixing times of systematic scan and random updates are not "too" different (roughly within squares of each other).

First, following from a recent paper by Roberts and Rosenthal, the authors construct several examples which do not satisfy the commonly held belief that systematic scan is never more than a constant factor slower and a log factor faster than random scan. The authors then provide a result Theorem 1 which provides weaker bounds, which however they verify at least under some conditions. In fact the Theorem compares random scan to a lazy version of the systematic scan and shows that and obtains bounds in terms of various other quantities, like the minimum probability, or the minimum holding probability.

MCMC is at the heart of many applications of modern machine learning and statistics. It is thus important to understand the computational and theoretical performance under various conditions. The present paper focused on examining systematic Gibbs sampling in comparison to random scan Gibbs. They do so first though the construction of several examples which challenge the dominant intuitions about mixing times, and develop theoretical bounds which are much wider than previously conjectured.