Archive for minorisation

sticky Metropolis

Posted in Statistics, University life with tags , , , , , , on September 6, 2013 by xi'an

My former student Roberto Casarin and his colleagues wrote (and arXived) a paper entitled Adaptive sticky generalized Metropolis algorithm. The basic idea is to use some of the rejected and past values of the chain to build an adaptive proposal, the criterion for choosing those values being related with the distance at the rejected point between the target and the proposal. In a sense, it gives a reward to surprising points, i.e. points where the proposal does poorly in approximating the target. On top of this, they include a multiple-try strategy where several values are generated from the current proposal and one of them is selected, to be accepted or rejected in a Metropolis step. The learning set may include several of the proposed (and rejected) values. This paper generalises Holden, Hauge and Holden (AoAP, 2009) and extends their proof of stationarity. The authors explore at length (the paper is 63 pages long!) the construction of the adaptive proposal distribution. This construction appears to be quite similar to Gilks’ and Wild’s (1993) ARMS algorithm. Hence, unless I missed a generalisation, it seems to me that the solutions are restricted to unidimensional settings. For instance, the authors propose to implement their algorithm for each complex conditional in a Gibbs sampler, meaning starting from scratch and running a large enough number of iterations to “reach” convergence. I also wonder at the correspondence between this construction and the original assumption of a minorisation condition wrt the target density in the event of an unbounded support. While this paper represents an interesting extension of the automated simulation algorithms of the ARMS type, and while the method is investigated thoroughly by several simulation experiments (in the second half of the paper), I remain somehow circumspect at the possibly of using ASMTM in complex high-dimensional problems as the learning cost soar with the dimension.

adaptive Metropolis-Hastings sampling using reversible dependent mixture proposals

Posted in Statistics with tags , , , , , on May 23, 2013 by xi'an

In the plane to Birmingham, I was reading this recent arXived paper by Minh-Ngoc Tran, Michael K. Pitt, and Robert Kohn. The adaptive structure of their ACMH algorithm is based upon two parallel Markov chains, the former (called the trial chain) feeding the proposal densities of the later (called the main chain), bypassing the more traditional diminishing adaptation conditions. (Even though convergence actually follows from a minorisation condition.) These proposals are mixtures of t distributions fitted by variational Bayes approximations. Furthermore, the proposals are (a) reversible and (b) mixing local [dependent] and global [independent] components. One nice aspect of the reversibility is that the proposals do not have to be evaluated at each step.

The convergence results in the paper indeed assume a uniform minorisation condition on all proposal densities: although this sounded restrictive at first (but allows for straightforward proofs), I realised this could be implemented by adding a specific component to the mixture as in Corollary 3. (I checked the proof to realise that the minorisation on the proposal extends to the minorisation on the Metropolis-Hastings transition kernel.) A reversible kernel is defined as satisfying the detailed balance condition, which means that a single Gibbs step is reversible even though the Gibbs sampler as a whole is not. If a reversible Markov kernel with stationary distribution ζ is used, the acceptance probability in the Metropolis-Hastings transition is

α(x,z) = min{1,π(z)ζ(x)/π(x)ζ(z)}

(a result I thought was already known). The sweet deal is that the transition kernel involves Dirac masses, but the acceptance probability bypasses the difficulty. The way mixtures of t distributions can be reversible follows from Pitt & Walker (2006) construction, with  ζ  a specific mixture of t distributions. This target is estimated by variational Bayes. The paper further bypasses my classical objection to the use of normal, t or mixtures thereof, distributions:  this modelling assumes a sort of common Euclidean space for all components, which is (a) highly restrictive and (b) very inefficient in terms of acceptance rate. Instead, Tran & al. resort to Metropolis-within-Gibbs by constructing a partition of the components into subgroups.

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