reading classics (#6)
Today my student Xiaolin Cheng presented the mythical 1990 JASA paper of Alan Gelfand and Adrian Smith, Sampling-based approaches to calculating marginal densities. The very one that started the MCMC revolution of the 1990′s! Re-reading it through his eyes was quite enlightening, even though he stuck quite closely to the paper. (To the point of not running his own simulation, nor even reporting Gelfand and Smith’s, as shown by the slides below. This would have helped, I think…)
Indeed, those slides focus very much on the idea that such substitution samplers can provide parametric approximations to the marginal densities of the components of the simulated parameters. To the point of resorting to importance sampling as an alternative to the standard Rao-Blackwell estimate, a solution that did not survive long. (We briefly discussed this point during the seminar, as the importance function was itself based on a Rao-Blackwell estimate, with possibly tail issues. Gelfand and Smith actually conclude on the higher efficiency of the Gibbs sampler.) Maybe not so surprisingly, the approximation of the “other” marginal, namely the marginal likelihood, as it is much more involved (and would lead to the introduction of the infamous harmonic mean estimator a few years later! And Chib’s (1995), which is very close in spirit to the Gibbs sampler). While Xiaolin never mentioned Markov chains in his talk, Gelfand and Smith only report that Gibbs sampling is a Markovian scheme, and refer to both Geman and Geman (1984) and Tanner and Wong (1987), for convergence issues. Rather than directly invoking Markov arguments as in Tierney (1994) and others. A fact that I find quite interesting, a posteriori, as it highlights the strong impact Meyn and Tweedie would have, three years later.