day four at ISBA 22

Woke up an hour later today! Which left me time to work on [shortening] my slides for tomorrow, run to Mon(t) Royal, and bike to St-Viateur Bagels for freshly baked bagels. (Which seemed to be missing salt, despite my low tolerance for salt in general.)

Terrific plenary lecture by Pierre Jacob in his Susie Bayarri’s Lecture about cut models!  Offering a very complete picture of the reasons for seeking modularisation, the theoretical and practical difficulties with the approach, and some asymptotics as well. Followed a great discussion by Judith on cut posteriors separating interest parameters from nuisance parameters, especially in semi-parametric models. Even introducing two priors on the same parameters! And by Jim Berger, who coauthored with Susie the major cut paper inspiring this work, and illustrated the concept on computer experiments (not falling into the fallacy pointed out by Martin Plummer at MCMski(v) in Chamonix!).

Speaking of which, the Scientific Committee for the incoming BayesComp²³ in Levi, Finland, had a working meeting to which I participated towards building the programme as it is getting near. For those interested in building a session, they should make preparations and take advantage of being together in Mon(t)réal, as the call is coming out pretty soon!

Attended a session on divide-and-conquer methods for dependent data, with Sanvesh Srivastava considering the case of hidden Markov models and block processing the observed sequence. Which is sort of justified by the forgettability of long-past observations. I wonder if better performances could be achieved otherwise as the data on a given time interval gives essentially information on the hidden chain at other time periods.

I was informed this morn that Jackie Wong, one speaker in our session tomorrow could not make it to Mon(t)réal for visa reasons. Which is unfortunate for him, the audience and everyone involved in the organisation. This reinforces my call for all-time hybrid conferences that avoid penalising (or even discriminating) against participants who cannot physically attend for ethical, political (visa), travel, health, financial, parental, or any other, reasons… I am often opposed the drawbacks of lower attendance, risk of a deficit, dilution of the community, but there are answers to those, existing or to be invented, and the huge audience at ISBA demonstrates a need for “real” meetings that could be made more inclusive by mirror (low-key low-cost) meetings.

Finished the day at Isle de Garde with a Pu Ehr flavoured beer, in a particularly lively (if not jazzy) part of the city…

day three at ISBA 22

Still woke up early too early [to remain operational for the poster session], finalised the selection of our MASH 2022/3 students, then returned to the Jean-Drapeau pool, which was  even more enjoyable in a crisp bright blue morning (and hardly anyone in my lane).

Attended a talk by Li Ma, who reviewed complexifying stick-breaking priors on the weights and introduced a balanced tree stick mechanism (why same depth?) (with links to Jara & Hanson 2010 and Stefanucci & Canale 2021). Then I listened to Giovanni Rebaubo creating clustering Gibbs-type processes along graphs, I sorted of dozed and missed the point as it felt as if the graph turned from a conceptual connection into a physical one! Catherine Forbes talked about a sequential version of stochastic variational approximation (published in St&Co) exploiting the update-one-at-a-time feature of Bayesian construction, except that each step relies on the previous approximation, meaning that the final—if fin there is!—approximation can end up far away from the optimal stochastic variational approximation. Assessing the divergence away from the target (in real time and tight budget would be nice).

After a quick lunch where I tasted seaweed-shell gyozas (!), I went to the generalised Bayesian inference session on Gibbs posteriors, [sort of] making up for the missed SAVI workshop! With Alice Kirichenko (Warwick) deriving information complexity bounds under misspecification, plus deriving an optimal value for the [vexing] coefficient η [in the Gibbs posterior], and Jack Jewson (ex-Warwick), raising the issue of improper models within Gibbs posteriors, although the reference or dominating measure is a priori arbitrary in these settings. But missing the third talk, about Gibbs posteriors again, and Chris Homes’ discussion, to attend part of the Savage (thesis) Award, with finalists Marta Catalano (Warwick faculty), Aditi Shenvi (Warwick student), and John O’Leary (an academic grand-children of mine’s as Pierre Jacob was his advisor). What a disappointment to have to wait for Friday night to hear the outcome!

I must confess to some  (French-speaker) énervement at hearing Mon(t)-réal massacred as Mon-t-real..! A very minor hindrance though, when put in perspective with my friend and Warwick colleague Gareth Roberts forced to evacuate his hotel last night due to a fire in basement, fortunately unscathed but ruining Day 3 for him… (Making me realise the conference hotel itself underwent a similar event 14 years ago.)

rope tricks

a week of fires

Last week, in a freak coincidence, both my 81-year old aunt, who lives near Hyères in the South of France, and my 79-year old next-door neighbour suffered from a home fire, alas destroying most of her house for my aunt and fortunately remaining confined to his garage for my neighbour, thanks to the quick and super-efficient intervention of the local fire brigade which did much better than our weak attempt at hosing down the flames. At least, no one got hurt in either case, but these close crises reminded me how quickly even a small fire can turn out of control…

Notre-Dame-de-Paris analysed by Andrew [not a book review]

As reported in Le Monde, Alexander van Geen, Yuling Yao, Tyler Ellis, and Andrew Gelman wrote a paper analysing the impact of the destruction of Notre-Dame last year in terms of lead concentration in the ground. As 460 tons of lead from the roof melted overnight. Based on  100 samples of surface soil collected by one author (not Andrew!) from tree pits, parks, and other sites in all directions within 1 km of the cathedral. Here is a plain language summary of the findings.

“This study attempts to estimate the extent to which the population of Paris was exposed to lead as a result of the Notre‐Dame cathedral fire of April 15, 2019. The concern stems from the large quantity of lead that covered the cathedral, some of which was injected into the air by the fire for several hours. In order to evaluate how much lead rising from the fire was redeposited nearby, surface soil samples were collected in all directions within a 1 km radius of the cathedral. Elevated levels of lead observed downwind of the cathedral indicate that surface soil preserved the mark of lead fallout from the fire. Although the estimated amount of lead redeposited within 1 km corresponds to only a small fraction of the total covering the cathedral, it could have posed a health hazard to children located downwind for a limited amount of time. Environmental testing on a larger scale immediately after the fire could have provided a more timely assessment of the scale of the problem and resulted in more pointed advice to the surrounding population on how to limit exposure to the fallout of lead.”

The statistical modelling is one of a spatial pattern of the lead distribution, using a mean-zero Gaussian process prior. And of a discretisation of the neighbourhood of the cathedral into uniform 30×30 locations. Without any further input, the model identifies properly the direction of the wind on that fateful evening. And logically concludes to a higher exposure than measured weeks after the fire. (Minor quibbles: a bias in self-declared test toward “a more educated, wealthier segment of the population” is unlikely in the immediate neighbourhood of Notre-Dame where the average flat sells at 16,000 euros per m², and the LCPP (Laboratoire Central de la Préfecture de Police) is not affiliated with the City of Paris but the Ministry of the Interior.)