Archive for maquis

ISBA 2016 [#6]

Posted in Kids, Mountains, pictures, Statistics, Travel, University life, Wines with tags , , , , , , , , , , , , , , on June 19, 2016 by xi'an

Fifth and final day of ISBA 2016, which was as full and intense as the previous ones. (Or even more if taking into account the late evening social activities pursued by most participants.) First thing in the morning, I managed to get very close to a hill top, thanks to the hints provided by Jeff Miller!, and with no further scratches from the nasty local thorn bushes. And I was back with plenty of time for a Bayesian robustness session with great talks. (Session organised by Judith Rousseau whom I crossed while running, rushing to the airport thanks to an Air France last-minute cancellation.) First talk by James Watson (on his paper with Chris Holmes on Kullback neighbourhoods on priors that Judith and I discussed recently in Statistical Science). Then as a contrapunto Peter Grünwald gave a neat geometric motivation for possible misbehaviour of Bayesian inference in non-convex misspecified environments and discussed his SafeBayes resolution that weights down the likelihood. In a sort of PAC-Bayesian way. And Erlis Ruli presented the ABC-R approach he developed with Laura Ventura and Nicola Sartori based on M-estimators and score functions. Making wonder [idly, as usual] whether cumulating different M-estimators would make a difference in the performances of the ABC algorithm.

David Dunson delivered one of the plenary lectures on high-dimensional discrete parameter estimation, including for instance categorical data. This wide-range talk covered many aspects and papers of David’s work, including a use of tensors I had neither seen nor heard of before before. With sparse modelling to resist the combinatoric explosion of contingency tables. However, and you may blame my Gallic pessimistic daemon for this remark, I have trouble to picture the meaning and relevance of a joint distribution on a space of hundreds and hundreds of dimension and similarly the ability to check the adequacy of any modelling in terms of goodness of fit. For instance, to borrow a non-military example from David’s talk, handling genetic data on ACGT sequences to infer its distribution sounds unreasonable unless most of the bases are mono-allelic. And the only way I see to test the realism of a model in this framework would be to engineer realisations of this distribution to observe the outcome, a test that seems neither feasible not desirable. Prediction based on such models may obviously operate satisfactorily without such realism requirements.

My first afternoon session (after the ISBA assembly that announced the location of ISBA 2020 in Yunnan, China!, home of Pu’ Ehr tea) was about accelerated MCMC schemes with talks by Sanvesh Srivastava on divide-and-conquer MCMC using Wasserstein barycentres, already discussed here, Minsuk Shin on a faster stochastic search variable selection which I could not understand, and Alex Beskos on the extension of Giles’ multilevel Monte Carlo to MCMC settings, which sounded worth investigating further even though I did not follow the notion all the way through. After listening to Luke Bornn explaining how to recalibrate grid data for climate science by accounting for correlation (with the fun title of `lost moments’), I rushed to my rental to [help] cook dinner for friends and… the ISBA 2016 conference was over!

ISBA 2016 [#5]

Posted in Mountains, pictures, Running, Statistics, Travel with tags , , , , , , , , , , , , , on June 18, 2016 by xi'an

from above Forte Village, Santa Magherita di Pula, Sardinia, June 17, 2016On Thursday, I started the day by a rather masochist run to the nearby hills, not only because of the very hour but also because, by following rabbit trails that were not intended for my size, I ended up being scratched by thorns and bramble all over!, but also with neat views of the coast around Pula.  From there, it was all downhill [joke]. The first morning talk I attended was by Paul Fearnhead and about efficient change point estimation (which is an NP hard problem or close to). The method relies on dynamic programming [which reminded me of one of my earliest Pascal codes about optimising a dam debit]. From my spectator’s perspective, I wonder[ed] at easier models, from Lasso optimisation to spline modelling followed by testing equality between bits. Later that morning, James Scott delivered the first Bayarri Lecture, created in honour of our friend Susie who passed away between the previous ISBA meeting and this one. James gave an impressive coverage of regularisation through three complex models, with the [hopefully not degraded by my translation] message that we should [as Bayesians] focus on important parts of those models and use non-Bayesian tools like regularisation. I can understand the practical constraints for doing so, but optimisation leads us away from a Bayesian handling of inference problems, by removing the ascertainment of uncertainty…

Later in the afternoon, I took part in the Bayesian foundations session, discussing the shortcomings of the Bayes factor and suggesting the use of mixtures instead. With rebuttals from [friends in] the audience!

This session also included a talk by Victor Peña and Jim Berger analysing and answering the recent criticisms of the Likelihood principle. I am not sure this answer will convince the critics, but I won’t comment further as I now see the debate as resulting from a vague notion of inference in Birnbaum‘s expression of the principle. Jan Hannig gave another foundation talk introducing fiducial distributions (a.k.a., Fisher’s Bayesian mimicry) but failing to provide a foundational argument for replacing Bayesian modelling. (Obviously, I am definitely prejudiced in this regard.)

The last session of the day was sponsored by BayesComp and saw talks by Natesh Pillai, Pierre Jacob, and Eric Xing. Natesh talked about his paper on accelerated MCMC recently published in JASA. Which surprisingly did not get discussed here, but would definitely deserve to be! As hopefully corrected within a few days, when I recoved from conference burnout!!! Pierre Jacob presented a work we are currently completing with Chris Holmes and Lawrence Murray on modularisation, inspired from the cut problem (as exposed by Plummer at MCMski IV in Chamonix). And Eric Xing spoke about embarrassingly parallel solutions, discussed a while ago here.