The data augmentation (da) approach to approximate sampling from an intractable probability density f(X) is based on the construction of a joint density, f(X, Y), whose conditional densities, f(X|Y) and f(Y|X), can be...
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The data augmentation (da) approach to approximate sampling from an intractable probability density f(X) is based on the construction of a joint density, f(X, Y), whose conditional densities, f(X|Y) and f(Y|X), can be straightforwardly sampled. However, many applications of the daalgorithm do not fall in this single-block setup. In these applications, X is partitioned into two components, X = (U, V), in such a way that it is easy to sample from f(Y|X), f(U|V, Y), and f(V|U, Y). We refer to this alternative version of da, which is effectively a three-variable Gibbs sampler, as two-blockda. We develop two methods to improve the performance of the daalgorithm in the two-block setup. These methods are motivated by the Haar PX-daalgorithm, which has been developed in previous literature to improve the performance of the single-blockdaalgorithm. The Haar PX-daalgorithm, which adds a computationally inexpensive extra step in each iteration of the daalgorithm while preserving the stationary density, has been shown to be optimal among similar techniques. However, as we illustrate, the Haar PX-daalgorithm does not lead to the required stationary density f(X) in the two-block setup. Our methods incorporate suitable generalizations and modifications to this approach, and work in the two-block setup. A theoretical comparison of our methods to the two-block da algorithm, a much harder task than the single-block setup due to nonreversibility and structural complexities, is provided. We successfully apply our methods to applications of the two-block da algorithm in Bayesian robit regression and Bayesian quantile regression. Supplementary materials for this article are available online.
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