ISBA 2016 [#6]

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!

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