**I**n connection with the official launch of the Alan Turing Institute (or ATI, of which Warwick is a partner), it funded an ATI Scoping workshop ~~yesterday~~ a week ago in Warwick around the notion(s) of intractable likelihood(s) and how this could/should fit within the themes of the Institute [hence the scoping]. This is one among many such scoping workshops taking place at all partners, as reported on the ATI website. Workshop that was quite relaxed and great fun, if only for getting together with most people (and friends) in the UK interested in the topic. But also pointing out some new themes I had not previously though of as related to ilike. For instance, questioning the relevance of likelihood for inference and putting forward decision theory under model misspecification, connecting with privacy and ethics [hence making intractable “good”!], introducing uncertain likelihood, getting more into network models, RKHS as a natural summary statistic, swarm of solutions for consensus inference… (And thanks to Mark Girolami for this homage to the iconic LP of the Sex Pistols!, that I played maniacally all over 1978…) My own two-cents into the discussion were mostly variations of other discussions, borrowing from ABC (and ABC slides) to call for a novel approach to approximate inference:

## Archive for intractable likelihood

## intractable likelihoods (even) for Alan

Posted in Kids, pictures, Statistics with tags ABC, Alan Turing Institute, consensus, decision theory, intractable likelihood, likelihood function, misspecified model, network, privacy, RKHS, Sex Pistols, summary statistics, University of Warwick on November 19, 2015 by xi'an## Non-reversible Markov Chains for Monte Carlo sampling

Posted in pictures, Statistics, Travel, University life with tags ABC, Alan Turing Institute, CRiSM, Hamiltonian Monte Carlo, intractable likelihood, lifting, Monte Carlo Statistical Methods, non-reversible diffusion, NUTS, overdamped Langevin algorithm, random walk, University of Warwick, workshop on September 24, 2015 by xi'an**T**his “week in Warwick” was not chosen at random as I was aware there is a workshop on non-reversible MCMC going on. (Even though CRiSM sponsored so many workshops in September that almost any week would have worked for the above sentence!) It has always been kind of a mystery to me that non-reversibility could make a massive difference in practice, even though I am quite aware that it does. And I can grasp some of the theoretical arguments why it does. So it was quite rewarding to sit in this Warwick amphitheatre and learn about overdamped Langevin algorithms and other non-reversible diffusions, to see results where convergence times moved from n to √n, and to grasp some of the appeal of lifting albeit in finite state spaces. Plus, the cartoon presentation of Hamiltonian Monte Carlo by Michael Betancourt was a great moment, not only because of the satellite bursting into flames on the screen but also because it gave a very welcome intuition about why reversibility was inefficient and HMC appealing. So I am grateful to my two colleagues, Joris Bierkens and Gareth Roberts, for organising this exciting workshop, with a most profitable scheduling favouring long and few talks. My next visit to Warwick will also coincide with a workshop on intractable likelihood, next November. This time part of the new Alan Turing Institute programme.

## MCMskv, Lenzerheide, 4-7 Jan., 2016 [news #1]

Posted in Mountains, Kids, Statistics, University life, Travel, pictures, R with tags ABC, ISBA, Monte Carlo Statistical Methods, MCMSki, Bayesian computation, Hamiltonian Monte Carlo, Zurich, Chamonix, Switzerland, Richard Tweedie, STAN, BayesComp, intractable likelihood, Blossom skis, Glenlivet, ski town, MCMskv, Lenzerheide, IDIDS on July 20, 2015 by xi'an**T**he BayesComp MCMski V [or MCMskv for short] has now its official website, once again maintained by Merrill Lietchy from Drexel University, Philadelphia, and registration is even open! The call for contributed sessions is now over, while the call for posters remains open until the very end. The novelty from the previous post is that there will be a “Breaking news” [in-between the Late news sessions at JSM and the crash poster talks at machine-learning conferences] session to highlight major advances among poster submissions. *And* that there will be an opening talk by Steve [the Bayesian] Scott on the 4th, about the frightening prospect of MCMC death!, followed by a round-table and a welcome reception, sponsored by the Swiss Supercomputing Centre. Hence the change in dates. Which still allows for arrivals in Zürich on the January 4th [be with you].

## Hamming Ball Sampler

Posted in Statistics, University life, Books with tags MCMC, simulation, auxiliary variable, intractable likelihood, Hamming distance, error correcting codes on May 7, 2015 by xi'an**M**ichalis Titsias and Christopher Yau just arXived a paper entitled the Hamming Ball sampler. Aimed at large and complex discrete latent variable models. The completion method is called after Richard Hamming, who is associated with code correcting methods (reminding me of one of the Master courses I took on coding, 30 years ago…), because it uses the Hamming distance in a discrete version of the slice sampler. One of the reasons for this proposal is that conditioning upon the auxiliary slice variable allows for the derivation of normalisation constants otherwise unavailable. The method still needs some calibration in the choice of blocks that partition the auxiliary variable and in the size of the ball. One of the examples assessed in the paper is a variable selection problem with 1200 covariates, out of which only 2 are relevant, while another example deals with a factorial HMM, involving 10 hidden chains. Since the paper compares each example with the corresponding block Gibbs sampling solution, it means this Gibbs sampling version is not intractable. It would be interesting to see a case where the alternative is not available…

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

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

## recycling accept-reject rejections

Posted in Statistics, University life with tags arXiv, Rao-Blackwellisation, accept-reject algorithm, Data augmentation, Monte Carlo Statistical Methods, George Casella, auxiliary variable, intractable likelihood, untractable normalizing constant, recycling 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, University life, Travel with tags workshop, University of Warwick, University of Oxford, intractable likelihood, i-like, University of Nottingham 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.)