Archive for Polya urn

21w5107 [day 2]

Posted in Books, Mountains, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , , , , , , , , on December 1, 2021 by xi'an

After a rich and local (if freezing) dinner on a rooftop facing the baroque Oaxaca cathedral, and an early invigorating outdoor swim in my case!, the morning session was mostly on mixtures, with Helen Ogden exploring X validation for (estimating the number k of components for) finite mixtures, when using the likelihood as an objective function. I was unclear of the goal however when considering that the data supporting the study was Uniform (0,1), nothing like a mixture of Normal distributions. And about the consistency attached to the objective function. The session ended with Diana Cai presenting a counter-argument in the sense that she proved, along with Trevor Campbell and Tamara Broderick, that the posterior on k diverges to infinity with the number n of observations if a mixture model is misspecified for said data. Which does not come as a major surprise since there is no properly defined value of k when the data is not generated from the adopted mixture. I would love to see an extension to the case when the k component mixture contains a non-parametric component! In-between, Alexander Ly discussed Bayes factors for multiple datasets, with some asymptotics showing consistency for some (improper!) priors if one sample size grows to infinity. With actually attaining the same rate under both hypotheses. Luis Nieto-Barajas presented an approach on uncertainty assessment through KL divergence for random probability measures, which requires a calibration of the KL in this setting, as KL does not enjoy a uniform scale, and a prior on a Pólya tree. And Chris Holmes presented a recent work with Edwin Fong and Steven Walker on a prediction approach to Bayesian inference. Which I had had on my reading list for a while. It is a very original proposal where likelihoods and priors are replaced by the sequence of posterior predictives and only parameters of interest get simulated. The Bayesian flavour of the approach is delicate to assess though, albeit a form of non-parametric Bayesian perspective… (I still need to read the paper carefully.)

In the afternoon session, Judith Rousseau presented her recent foray in cut posteriors for semi-parametric HMMs. With interesting outcomes for efficiently estimating the transition matrix, the component distributions, and the smoothing distribution. I wonder at the connection with safe Bayes in that cut posteriors induce a loss of information. Sinead Williamson spoke on distributed MCMC for BNP. Going back at the “theme of the day”, namely clustering and finding the correct (?) number of clusters. With a collapsed versus uncollapsed division that reminded me of the marginal vs. conditional María Gil-Leyva discussed yesterday. Plus a decomposition of a random measure into a finite mixture and an infinite one that also reminded me of the morning talk of Diana Cai. (And making me wonder at the choice of the number K of terms in the finite part.) Michele Guindani spoke about clustering distributions (with firecrackers as a background!). Using the nDP mixture model, which was show to suffer from degeneracy (as discussed by Frederico Camerlenghi et al. in BA). The subtle difference stands in using the same (common) atoms in all random distributions at the top of the hierarchy, with independent weights. Making the partitions partially exchangeable. The approach relies on Sylvia’s generalised mixtures of finite mixtures. With interesting applications to microbiome and calcium imaging (including a mice brain in action!). And Giovanni Rebaudo presented a generalised notion of clustering aligned on a graph, with some observations located between the nodes corresponding to clusters. Represented as a random measure with common parameters for the clusters and separated parameters outside. Interestingly playing on random partitions, Pólya urns, and species sampling.

ABC in Svalbard [#1]

Posted in Books, Mountains, pictures, R, Running, Statistics, Travel, University life with tags , , , , , , , , , , , , , , , , , , , , , , , , , on April 13, 2021 by xi'an

It started a bit awkwardly for me as I ran late, having accidentally switched to UK time the previous evening (despite a record-breaking biking-time to the University!), then the welcome desk could not find the key to the webinar room and I ended up following the first session from my office, by myself (and my teapot)… Until we managed to reunite in the said room (with an air quality detector!).

Software sessions are rather difficult to follow and I wonder what the idea on-line version should be. We could borrow from our teaching experience new-gained from the past year, where we had to engage students without the ability to roam the computer lab and look at their screens to force engage them into coding. It is however unrealistic to run a computer lab, unless a few “guinea pigs” could be selected in advance and show their progress or lack thereof during the session. In any case, thanks to the speakers who made the presentations of

  1. BSL(R)
  2. ELFI (Python)
  3. ABCpy (Python)

this morning/evening. (Just taking the opportunity to point out the publication of the latest version of DIYABC!).

Florence Forbes’ talk on using mixture of experts was quite alluring (and generated online discussions during the break, recovering some of the fun in real conferences), esp. from my longtime interest normalising flows in mixtures of regression (and more to come as part of our biweekly reading group!). Louis talked about gaining efficiency by not resampling the entire data in large network models. Edwin Fong brought martingales and infinite dimension distributions to the rescue, generalising Polya urns! And Justin Alsing discussed the advantages of estimating the likelihood rather than estimating the posterior, which sounds counterintuitive. With a return to mixtures as approximations, using instead normalising flows. With the worth-repeating message that ABC marginalises over nuisance parameters so easily! And a nice perspective on ABayesian decision, which does not occur that often in the ABC literature. Cecilia Viscardi made a link between likelihood estimation and large deviations à la Sanov, the rare event being associated with the larger distances, albeit dependent on a primary choice of the tolerance. Michael Gutmann presented an intringuing optimisation Monte Carlo approach from his last year AISTATS 2020 paper, the simulated parameter being defined by a fiducial inversion. Reweighted by the prior times a Jacobian term, which stroke me as a wee bit odd, ie using two distributions on θ. And Rito concluded the day by seeking approximate sufficient statistics by constructing exponential families whose components are themselves parameterised as neural networks with neural parameter ω. Leading to an unnormalised model because of the energy function, hence to the use of inference techniques on ω that do not require the constant, like Gutmann & Hyvärinen (2012). And using the (pseudo-)sufficient statistic as ABCsummary statistic. Which still requires an exchange MCMC step within ABC.

assessing MCMC convergence

Posted in Books, Statistics, University life with tags , , , , , , , , , , , on June 6, 2019 by xi'an

When MCMC became mainstream in the 1990’s, there was a flurry of proposals to check, assess, and even guarantee convergence to the stationary distribution, as discussed in our MCMC book. Along with Chantal Guihenneuc and Kerrie Mengersen, we also maintained for a while a reviewww webpage categorising theses. Niloy Biswas and Pierre Jacob have recently posted a paper where they propose the use of couplings (and unbiased MCMC) towards deriving bounds on different metrics between the target and the current distribution of the Markov chain. Two chains are created from a given kernel and coupled with a lag of L, meaning that after a while, the two chains become one with a time difference of L. (The supplementary material contains many details on how to induce coupling.) The distance to the target can then be bounded by a sum of distances between the two chains until they merge. The above picture from the paper is a comparison a Polya-Urn sampler with several HMC samplers for a logistic target (not involving the Pima Indian dataset!). The larger the lag L the more accurate the bound. But the larger the lag the more expensive the assessment of how many steps are needed to convergence. Especially when considering that the evaluation requires restarting the chains from scratch and rerunning until they couple again, rather than continuing one run which can only brings the chain closer to stationarity and to being distributed from the target. I thus wonder at the possibility of some Rao-Blackwellisation of the simulations used in this assessment (while realising once more than assessing convergence almost inevitably requires another order of magnitude than convergence itself!). Without a clear idea of how to do it… For instance, keeping the values of the chain(s) at the time of coupling is not directly helpful to create a sample from the target since they are not distributed from that target.

[Pierre also wrote a blog post about the paper on Statisfaction that is definitely much clearer and pedagogical than the above.]

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