## stats postdoc in Barcelona

Posted in Statistics, Travel, University life with tags , , , , , , , , , , , , on April 5, 2022 by xi'an

Here is a call for exciting postdoc positions in high-dimensional statistics for network & graphical models analysis, Barcelona:

We have two post-doctoral positions to work on a range of topics related to high-dimensional statistics for networks, ultra high-dimensional graphical models, frameworks linking networks and graphical models, multivariate structural learning and time series analysis. The positions are for 1 year, with a potential for extension to 18 months, with a gross yearly salary of 40,000€. The scope is ample and allows for sub-projects related to mathematical statistics and statistical learning theory, data analysis methodology in penalized likelihood and Bayesian statistics, and computational methods. The positions are related to a Huawei grant, which also offers opportunities to explore applications of the developed theory & methods.

The project is primarily hosted by the Statistics group at UPF and the BSE Data Science Center in Barcelona (Spain), and is in collaboration with Luc Devroye at McGill University in Montreal (Canada) and Piotr Zwiernik at the University of Toronto (Canada). The primary supervisors are Christian Brownlees, Luc Devroye, Gábor Lugosi, David Rossell and Piotr Zwiernik, although collaborations with other professors of these research groups are also possible.

Interested candidates should send an updated CV and a short research statement to David Rossell (david.rossell AT upf.edu). They should ask 3 referees to send a letter of reference on their behalf.

The deadline for applying for the first position is April 30 2022, the deadline for the second position is June 15 2022.

## empirically Bayesian [wISBApedia]

Posted in Statistics with tags , , , , , , , on August 9, 2021 by xi'an

Last week I was pointed out a puzzling entry in the “empirical Bayes” Wikipedia page. The introduction section indeed contains a description of an iterative simulation method that involves an hyperprior $$p(η)$$even though the empirical Bayes perspective does not involve an hyperprior.

While the entry is vague and lacks formulae

These suggest an iterative scheme, qualitatively similar in structure to a Gibbs sampler, to evolve successively improved approximations to $$p(θ$$$$∣$$$$y)$$ and $$p(η∣y)$$. First, calculate an initial approximation to $$p(θ∣y)$$ ignoring the $$η$$ dependence completely; then calculate an approximation to $$p(η$$$$|$$$$y)$$ based upon the initial approximate distribution of $$p(θ$$$$∣$$$$y)$$; then use this $$p(η$$$$∣$$$$y)$$ to update the approximation for $$p(θ$$$$∣$$$$y)$$; then update $$p(η$$$$∣$$$$y)$$; and so on.

it sounds essentially equivalent to a Gibbs sampler, possibly a multiple try Gibbs sampler (unless the author had another notion in mind, alas impossible to guess since no reference is included).

Beyond this specific case, where I think the entire paragraph should be erased from the “empirical Bayes” Wikipedia page, I discussed the general problem of some poor Bayesian entries in Wikipedia with Robin Ryder, who came with the neat idea of running (collective) Wikipedia editing labs at ISBA conferences. If we could further give an ISBA label to these entries, as a certificate of “Bayesian orthodoxy” (!), it would be terrific!

## Bayeswashin

Posted in Books, pictures, Statistics, University life with tags , , , , , , , , , on May 16, 2021 by xi'an

Just heard via ISBA that the Cass Business School of City University London is changing its name to the Bayes Business School “after it was found (!) that some of Sir John Cass’s wealth was obtained through his links to the slave trade.” One of the school buildings is located on Bunhill Row, which leads to Bunhill Fields cemetery where Thomas Bayes (and Richard Price) were buried. And which stands near the Royal Statistical Society building.

“Bayes’ theorem suggests that we get closer to the truth by constantly updating our beliefs in proportion to the weight of new evidence. It is this idea – not only the person – that is the motivation behind adopting this name.”

While it is a notable recognition of the field that Thomas Bayes was selected by [some members of] the City community, I hope the name will not become a registered trademark! And ponder the relevance of naming schools, buildings, prizes, whatever after individuals who should remain mere mortals rather than carrying the larger-than-life burden of representing ideals and values. And the irony of having a business school named after someone who never worked, being financially wealthy by inheritance (from his Sheffield cutler ancestors).  Or of promoting diversity through a religious zealot leaning towards Arianism.

“In Bayes Business School, we believe we now have a name that reflects who we are and the values we hold. Even though Bayes lived a long time ago, his ideas and his name are very much connected to the future rather than the past.”

## mathematical theory of Bayesian statistics [book review]

Posted in Books, Statistics, Travel, University life with tags , , , , , , , , , , on May 6, 2021 by xi'an

I came by chance (and not by CHANCE) upon this 2018 CRC Press book by Sumio Watanabe and ordered it myself to gather which material it really covered. As the back-cover blurb was not particularly clear and the title sounded quite general. After reading it, I found out that this is a mathematical treatise on some aspects of Bayesian information criteria, in particular on the Widely Applicable Information Criterion (WAIC) that was introduced by the author in 2010. The result is a rather technical and highly focussed book with little motivation or intuition surrounding the mathematical results, which may make the reading arduous for readers. Some background on mathematical statistics and Bayesian inference is clearly preferable and the book cannot be used as a textbook for most audiences, as opposed to eg An Introduction to Bayesian Analysis by J.K. Ghosh et al. or even more to Principles of Uncertainty by J. Kadane. In connection with this remark the exercises found in the book are closer to the delivery of additional material than to textbook-style exercises.

“posterior distributions are often far from any normal distribution, showing that Bayesian estimation gives the more accurate inference than other estimation methods.”

The overall setting is one where both the sampling and the prior distributions are different from respective “true” distributions. Requiring a tool to assess the discrepancy when utilising a specific pair of such distributions. Especially when the posterior distribution cannot be approximated by a Normal distribution. (Lindley’s paradox makes an interesting incognito incursion on p.238.) The WAIC is supported for the determination of the “true” model, in opposition to AIC and DIC, incl. on a mixture example that reminded me of our eight versions of DIC paper. In the “Basic Bayesian Theory” chapter (§3), the “basic theorem of Bayesian statistics” (p.85) states that the various losses related with WAIC can be expressed as second-order Taylor expansions of some cumulant generating functions, with order o(n⁻¹), “even if the posterior distribution cannot be approximated by any normal distribution” (p.87). With the intuition that

“if a log density ratio function has a relatively finite variance then the generalization loss, the cross validation loss, the training loss and WAIC have the same asymptotic behaviors.”

Obviously, these “basic” aspects should come as a surprise to a fair percentage of Bayesians (in the sense of not being particularly basic). Myself included. Chapter 4 exposes why, for regular models, the posterior distribution accumulates in an ε neighbourhood of the optimal parameter at a speed O(n2/5). With the normalised partition function being of order n-d/2 in the neighbourhood and exponentially negligible outside. A consequence of this regular asymptotic theory is that all above losses are asymptotically equivalent to the negative log likelihood plus similar order n⁻¹ terms that can be ordered. Chapters 5 and 6 deal with “standard” [the likelihood ratio is a multi-index power of the parameter ω] and general posterior distributions that can be written as mixtures of standard distributions,  with expressions of the above losses in terms of new universal constants. Again, a rather remote concern of mine. The book also includes a chapter (§7) on MCMC, with a rather involved proof that a Metropolis algorithm satisfies detailed balance (p.210). The Gibbs sampling section contains an extensive example on a two-dimensional two-component unit-variance Normal mixture, with an unusual perspective on the posterior, which is considered as “singular” when the true means are close. (Label switching or the absence thereof is not mentioned.) In terms of approximating the normalising constant (or free energy), the only method discussed there is path sampling, with a cryptic remark about harmonic mean estimators (not identified as such). In a final knapsack chapter (§9),  Bayes factors (confusedly denoted as L(x)) are shown to be most powerful tests in a Bayesian sense when comparing hypotheses without prior weights on said hypotheses, while posterior probability ratios are the natural statistics for comparing models with prior weights on said models. (With Lindley’s paradox making another appearance, still incognito!) And a  notion of phase transition for hyperparameters is introduced, with the meaning of a radical change of behaviour at a critical value of said hyperparameter. For instance, for a simple normal- mixture outlier model, the critical value of the Beta hyperparameter is α=2. Which is a wee bit of a surprise when considering Rousseau and Mengersen (2011) since their bound for consistency was α=d/2.

In conclusion, this is quite an original perspective on Bayesian models, covering the somewhat unusual (and potentially controversial) issue of misspecified priors and centered on the use of information criteria. I find the book could have benefited from further editing as I noticed many typos and somewhat unusual sentences (at least unusual to me).

[Disclaimer about potential self-plagiarism: this post or an edited version should eventually appear in my Books Review section in CHANCE.]

## approximate Bayesian inference [survey]

Posted in Statistics with tags , , , , , , , , , , , , , , , , , , on May 3, 2021 by xi'an

In connection with the special issue of Entropy I mentioned a while ago, Pierre Alquier (formerly of CREST) has written an introduction to the topic of approximate Bayesian inference that is worth advertising (and freely-available as well). Its reference list is particularly relevant. (The deadline for submissions is 21 June,)