Archive for high dimensions

data assimilation and reduced modelling for high-D problems [CIRM]

Posted in Books, Kids, Mountains, pictures, Running, Statistics, University life with tags , , , , , , , , , , , , , , , , , on February 8, 2021 by xi'an

Next summer, from 19 July till 27 August, there will be a six week program at CIRM on the above theme, bringing together scientists from both the academic and industrial communities. The program includes a one-week summer school followed by 5 weeks of research sessions on projects proposed by academic and industrial partners.

Confirmed speakers of the summer school (Jul 19-23) are:

  • Albert Cohen (Sorbonne University)
  • Masoumeh Dashti (University of Sussex)
  • Eric Moulines (Ecole Polytechnique)
  • Anthony Nouy (Ecole Centrale de Nantes)
  • Claudia Schillings (Mannheim University)

Junior participants may apply for fellowships to cover part or the whole stay. Registration and application to fellowships will be open soon.

end-to-end Bayesian learning [CIRM]

Posted in Books, Kids, Mountains, pictures, Running, Statistics, University life with tags , , , , , , , , , , , , , , , , , on February 1, 2021 by xi'an

Next Fall, there will be a workshop at CIRM, Luminy, Marseilles, on Bayesian learning. It takes place 22-29 October 2021 on this wonderful campus at the border with the beautiful Parc National des Calanques, in a wonderfully renovated CIRM building and involves friends and colleagues of mine as organisers and plenary speakers. (I am not involved!, but plan to organise a scalable MCMC workshop there the year after!) The conference is well-supported and the housing fees will be minimal since the centre is also subsidized by CNRS. The deadline for contributed talks and posters is 22 March, while it is 15 June for registration. Hopefully by this time the horizon will have cleared up enough to consider traveling and meeting again. Hopefully. (In which case I will miss this wonderful conference due to other meeting and teaching commitments in the Fall.)

Gabriel’s talk at Warwick on optimal transport

Posted in Statistics with tags , , , , , , on March 4, 2020 by xi'an

ISBA2020 program

Posted in Kids, Statistics, Travel, University life with tags , , , , , , , , , , , , on January 29, 2020 by xi'an

The scheduled program for ISBA 2020 is now on-line. And full of exciting sessions, many with computational focus. With dear hopes that the nCo-2019 epidemics will have abated by then (and not solely for the sake of the conference, most obviously!). While early registration ends by 15 April, the deadline for junior travel support ends up this month. And so does the deadline for contributions.

A precursor of ABC-Gibbs

Posted in Books, R, Statistics with tags , , , , , , , , , , on June 7, 2019 by xi'an

Following our arXival of ABC-Gibbs, Dennis Prangle pointed out to us a 2016 paper by Athanasios Kousathanas, Christoph Leuenberger, Jonas Helfer, Mathieu Quinodoz, Matthieu Foll, and Daniel Wegmann, Likelihood-Free Inference in High-Dimensional Model, published in Genetics, Vol. 203, 893–904 in June 2016. This paper contains a version of ABC Gibbs where parameters are sequentially simulated from conditionals that depend on the data only through small dimension conditionally sufficient statistics. I had actually blogged about this paper in 2015 but since then completely forgotten about it. (The comments I had made at the time still hold, already pertaining to the coherence or lack thereof of the sampler. I had also forgotten I had run an experiment of an exact Gibbs sampler with incoherent conditionals, which then seemed to converge to something, if not the exact posterior.)

All ABC algorithms, including ABC-PaSS introduced here, require that statistics are sufficient for estimating the parameters of a given model. As mentioned above, parameter-wise sufficient statistics as required by ABC-PaSS are trivial to find for distributions of the exponential family. Since many population genetics models do not follow such distributions, sufficient statistics are known for the most simple models only. For more realistic models involving multiple populations or population size changes, only approximately-sufficient statistics can be found.

While Gibbs sampling is not mentioned in the paper, this is indeed a form of ABC-Gibbs, with the advantage of not facing convergence issues thanks to the sufficiency. The drawback being that this setting is restricted to exponential families and hence difficult to extrapolate to non-exponential distributions, as using almost-sufficient (or not) summary statistics leads to incompatible conditionals and thus jeopardise the convergence of the sampler. When thinking a wee bit more about the case treated by Kousathanas et al., I am actually uncertain about the validation of the sampler. When tolerance is equal to zero, this is not an issue as it reproduces the regular Gibbs sampler. Otherwise, each conditional ABC step amounts to introducing an auxiliary variable represented by the simulated summary statistic. Since the distribution of this summary statistic depends on more than the parameter for which it is sufficient, in general, it should also appear in the conditional distribution of other parameters. At least from this Gibbs perspective, it thus relies on incompatible conditionals, which makes the conditions proposed in our own paper the more relevant.