## Archive for University of Oxford

## snapshot from Oxford [#2]

Posted in Kids, pictures, Travel, University life with tags England, lawn, Magdalen College, University of Oxford, winter light on February 9, 2016 by xi'an## Oxford snapshot

Posted in pictures, Running, Travel, University life with tags England, Oxford, St. Edmund Hall, University of Oxford on February 2, 2016 by xi'an## métro static

Posted in Kids, Statistics, Travel with tags Bayesian tests of hypotheses, Birmingham, concentration inequalities, signal processing, SPA 2015, stochastic processes, University of Oxford on July 19, 2015 by xi'an*[In the train shuttle at Birmingham airport, two young guys, maybe back from SPA 2015, discussing signal processing:]*

– In Bayesian statistics, they use a different approach to testing hypotheses… You see, they put priors on the different hypotheses…

– But in the end it all boils down to concentration inequalities…

## Oxford snapshot

Posted in pictures, Travel, University life with tags architecture, biochemistry, England, summer, sunset, University of Oxford on July 18, 2015 by xi'an## SPA 2015 Oxford [my day #2]

Posted in pictures, Statistics, Travel, University life with tags British Rail, Keble College, Leamington Spa, Oxford, particle filters, pseudo-marginal MCMC, SPA 2015, systematic resampling, unbiased estimation, University of Oxford on July 17, 2015 by xi'an**T**oday I [barely made it on a delayed train from Leaminton Spa to Oxford as I] chaired my invited session at SPA 2015 on advanced MCMC methodology. The three speakers, Randal Douc, Mike Pitt and Matti Vihola, all gave talks related to the pseudo-marginal technique. For instance, Randal gave examples of guaranteed variance improvements by adding randomisation steps in the generation of the rv’s behind the unbiased estimation of the likelihood function. Mike Pitt presented the paper I discussed a little while ago about evaluating the computing performances of pseudo-marginal approximations, with a fairly compelling perspective [I may have missed from the paper] on approximating the distribution on the approximation to the log-likelihood as a normal. Which led me to ponder at the ultimate version where the log-likelihood itself would get directly simulated in an MCMC algorithm bypassing the preliminary simulation of the parameters. Sounds a bit too fantasy-like to be of any use… Matti Vihola also presented recent results with Christophe Andrieu on comparing pseudo-marginal approximations, based on convex ordering properties. They included a domination result on ABC-MCM algorithms, as noted in a recent post. Which made me musing about the overall importance of unbiasedness in the global picture, where all we need are converging approximations, *in fine*.

## SPA 2015 Oxford

Posted in pictures, Statistics, Travel, University life with tags ABC, ABC model choice, Bayesian Analyis, campus, CART, classification, JSM, model posterior probabilities, Montpellier, pine trees, posterior expected loss, random forests, SPA 2015, summary statistics, Université de Montpellier, University of Oxford on July 14, 2015 by xi'an**T**oday I gave a talk on Approximate Bayesian model choice via random forests at the yearly SPA (Stochastic Processes and their Applications) 2015 conference, taking place in Oxford (a nice town near Warwick) this year. In Keble College more precisely. The slides are below and while they are mostly repetitions of earlier slides, there is a not inconsequential novelty in the presentation, namely that I included our most recent and current perspective on ABC model choice. Indeed, when travelling to Montpellier two weeks ago, we realised that there was a way to solve our posterior probability conundrum!

Despite the heat wave that rolled all over France that week, we indeed figured out a way to estimate the posterior probability of the selected (MAP) model, way that we had deemed beyond our reach in previous versions of the talk and of the paper. The fact that we could not provide an estimate of this posterior probability and had to rely instead on a posterior expected loss was one of the arguments used by the PNAS reviewers in rejecting the paper. While the posterior expected loss remains a quantity worth approximating and reporting, the idea that stemmed from meeting together in Montpellier is that (i) the posterior probability of the MAP is actually related to another posterior loss, when conditioning on the observed summary statistics and (ii) this loss can be itself estimated via a random forest, since it is another function of the summary statistics. A posteriori, this sounds trivial but we had to have a new look at the problem to realise that using ABC samples was not the only way to produce an estimate of the posterior probability! (We are now working on the revision of the paper for resubmission within a few week… Hopefully before JSM!)