**F**ollowing the highly successful* [authorised opinion!, from objective sources]* MCMski IV, in Chamonix last year, the BayesComp section of ISBA has decided in favour of a two-year period, which means the great item of news that next year we will meet again for MCMski V [or MCMskv for short], this time on the snowy slopes of the Swiss town of Lenzerheide, south of Zürich. The committees are headed by the indefatigable Antonietta Mira and Mark Girolami. The plenary speakers have already been contacted and Steve Scott (Google), Steve Fienberg (CMU), David Dunson (Duke), Krys Latuszynski (Warwick), and Tony Lelièvre (Mines, Paris), have agreed to talk. Similarly, the nine invited sessions have been selected and will include Hamiltonian Monte Carlo, Algorithms for Intractable Problems (ABC included!), Theory of (Ultra)High-Dimensional Bayesian Computation, Bayesian NonParametrics, Bayesian Econometrics, Quasi Monte Carlo, Statistics of Deep Learning, Uncertainty Quantification in Mathematical Models, and Biostatistics. There will be afternoon tutorials, including a practical session from the Stan team, tutorials for which call is open, poster sessions, a conference dinner at which we will be entertained by the unstoppable Imposteriors. The Richard Tweedie ski race is back as well, with a pair of Blossom skis for the winner!

## Archive for intractable likelihood

## MCMskv, Lenzerheide, Jan. 5-7, 2016

Posted in Kids, Mountains, pictures, R, Statistics, Travel, University life with tags ABC, BayesComp, Bayesian computation, Blossom skis, Chamonix, Glenlivet, Hamiltonian Monte Carlo, intractable likelihood, ISBA, MCMSki, MCMskv, Monte Carlo Statistical Methods, Richard Tweedie, ski town, STAN, Switzerland, Zurich on March 31, 2015 by xi'an## recycling accept-reject rejections

Posted in Statistics, University life with tags accept-reject algorithm, arXiv, auxiliary variable, Data augmentation, George Casella, intractable likelihood, Monte Carlo Statistical Methods, Rao-Blackwellisation, recycling, untractable normalizing constant on July 1, 2014 by xi'an**V**inayak Rao, Lizhen Lin and David Dunson just arXived a paper which proposes anew technique to handle intractable normalising constants. And which exact title is Data augmentation for models based on rejection sampling. (Paper that I read in the morning plane to B’ham, since this is one of my weeks in Warwick.) The central idea therein is that, if the sample density (*aka* likelihood) satisfies

where all terms but p are known in closed form, then completion by the rejected values of an hypothetical accept-reject algorithm−hypothetical in the sense that the data does not have to be produced by an accept-reject scheme but simply the above domination condition to hold−allows for a data augmentation scheme. Without requiring the missing normalising constant. Since the completed likelihood is

A closed-form, if not necessarily congenial, function.

**N**ow this is quite a different use of the “rejected values” from the accept reject algorithm when compared with our 1996 Biometrika paper on the Rao-Blackwellisation of accept-reject schemes (which, still, could have been mentioned there… Or Section 4.2 of Monte Carlo Statistical Methods. Rather than re-deriving the joint density of the augmented sample, “accepted+rejected”.)

**I**t is a neat idea in that it completely bypasses the approximation of the normalising constant. And avoids the somewhat delicate tuning of the auxiliary solution of Moller et al. (2006) The difficulty with this algorithm is however in finding an upper bound M on the unnormalised density f that is

- in closed form;
- with a manageable and tight enough “constant” M;
- compatible with running a posterior simulation conditional on the added rejections.

The paper seems to assume further that the bound M is independent from the current parameter value θ, at least as suggested by the notation (and Theorem 2), but this is not in the least necessary for the validation of the formal algorithm. Such a constraint would pull M higher, hence reducing the efficiency of the method. Actually the matrix Langevin distribution considered in the first example involves a bound that depends on the parameter κ.

**T**he paper includes a result (Theorem 2) on the uniform ergodicity that relies on heavy assumptions on the proposal distribution. And a rather surprising one, namely that the probability of *rejection* is bounded from below, i.e. calling for a *less* efficient proposal. Now it seems to me that a uniform ergodicity result holds as well when the probability of *acceptance* is bounded from below since, then, the event when no rejection occurs constitutes an atom from the augmented Markov chain viewpoint. There therefore occurs a renewal each time the rejected variable set ϒ is empty, and ergodicity ensues (Robert, 1995, *Statistical Science*).

**N**ote also that, despite the opposition raised by the authors, the method *per se* does constitute a pseudo-marginal technique à la Andrieu-Roberts (2009) since the independent completion by the (pseudo) rejected variables produces an unbiased estimator of the likelihood. It would thus be of interest to see how the recent evaluation tools of Andrieu and Vihola can assess the loss in efficiency induced by this estimation of the likelihood.

*Maybe some further experimental evidence tomorrow…*

## i-like Oxford [workshop, March 20-21, 2014]

Posted in Statistics, Travel, University life with tags i-like, intractable likelihood, University of Nottingham, University of Oxford, University of Warwick, workshop on February 5, 2014 by xi'an**T**here will be another i-like workshop this Spring, over two days in Oxford, St Anne’s College, involving talks by Xiao-Li Meng and Eric Moulines, as well as by researchers from the participating universities. Registration is now open. (I will take part as a part-time participant, travelling from Nottingham where I give a seminar on the 20th.)

## Asymptotically Exact, Embarrassingly Parallel MCMC

Posted in Books, Statistics, University life with tags acceleration of MCMC algorithms, big data, intractable likelihood, nonparametric probability density estimation, parallel processing, parallelisation, subsampling on November 26, 2013 by xi'an**W**illie Neiswanger, Chong Wang, and Eric Xing (from CMU) recently arXived a paper entitled as above. The “embarrassing” in the title refers to the “embarrassingly simple” solution proposed therein, namely to solve the difficulty in handling very large datasets by running completely independent parallel MCMC samplers on parallel threads or computers and using the outcomes of those samplers as density estimates, pulled together as a product towards an approximation of the true posterior density. In other words, the idea is to break the posterior as

and to use the estimate

where the individual estimates are obtained by, say, non-parametric estimates. The method is then “asymptotically exact” in the weak (and unsurprising) sense of being converging in the number of MCMC iterations. Still, there is a theoretical justification that is not found in previous parallel methods that mixed all resulting samples without accounting for the subsampling. And I also appreciate the point that, in many cases, running MCMC samplers with subsamples produces faster convergence.

**I**n the paper, the division of *p* into its components is done by partitioning the iid data into m subsets. And taking a power 1/m of the prior in each case. (Which may induce improperness issues.) However, the subdivision is arbitrary and can thus be implemented in other cases than the fairly restrictive iid setting. Because each (subsample) non-parametric estimate involves T terms, the resulting overall estimate contains *T ^{m }*terms and the authors suggest using an independent Metropolis-within-Gibbs sampler to handle this complexity. Which is necessary [took me a while to realise this!] for producing a final sample from the (approximate) true posterior distribution. As an aside, I wonder why the bandwidths are all equal across all subsamples, as they should depend on those. And as it would not make much of a difference. It would also be interesting to build a typology of cases where subsampling leads to subposteriors that are close to orthogonal, preventing the implementation of the method.

**A**s it happened, I read this paper on the very day Nial Friel (University College Dublin) gave a seminar at the Big’MC seminar on the convergence of approximations to ergodic transition kernels, based on the recent results of Mitrophanov on the stability of Markov chains, where he introduced the topic by the case of datasets large enough to prevent the computation of the likelihood function.

## I like…intractable likelihoods (openings)

Posted in Statistics with tags ABC, EPSRC, i-like, intractable likelihood, Lancaster University, likelihood-free methods, openings, University of Bristol, University of Oxford, University of Warwick on December 22, 2012 by xi'an**A** new EPSRC programme grant, called **i-like**, has been awarded to researchers in Bristol, Lancaster, Oxford, and Warwick, to conduct research on intractable likelihoods. (I am also associated to this program as a [grateful] collaborator.) This covers several areas of statistics, like big data and inference on stochastic process, but my own primary interest in the programme is of course the possibilities to conduct collaboration on ABC and composite likelihood methods. (Great website design, by the way!)

**A** first announcement is that there will be a half-day launch in Oxford on January 31, 2013, which program is now available. Followed by a workshop in mid-May in Warwick (to which I will participate). This event is particularly aimed at PhD students and early-career researchers. The second announcement is that the EPSRC programme grant provides funding for five postdoctoral positions over a duration of four years, which is of course stupendous! So if you like i-like as much as I like it, and are a new researcher looking for opportunities in exciting areas, you should definitely consider applying!