Archive for CDG

BayesComp 2025.1

Posted in Running, Statistics, Travel, University life with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 18, 2025 by xi'an

Minus one day at BayesComp 2025! As I am attending the model misspecification satellite workshop (ten minutes late, due to repeated path finding protocol!), with an extended presentation by Jeremias Knoblauch on post-Bayesian inference, incl. powered likelihood and Gibbs posteriors. A very smooth and pedagogical presentation, esp. in the hybrid mode. A perspective I associate with the difficulties of making sense of the post-posterior, not truly a posterior, of calibrating the penalty (eg λ), picking the loss (α, β, γ divergences?) , and the drift towards learning goals since the new measure is the post-posterior predictive. Sort of paradoxical return to a Gaussian post-posterior on the parameter that does not seem to stay robust. Horrendous computational issues, when the loss itself is an integral. Use of the zig-zag sampler with an estimated unbiased gradient of the loss, much faster than pseudo-marginal, which (naïvely?) makes sense both because PDMPs directly use scores and because of the power of stochastic gradient methods. Worse perspectives for optimisation-centric posterior that are essentially vamped versions of GANs. For instance, what is the meaning of the coverage probabilities?

The second talk by Jonathan Huggins was on DC (not bagged) posteriors as martingale posteriors with m<∞ (approximating marginal distributions with random kernel MCMC—which persists in simulating the marginalised or integrated variable u from its prior, rather than adapting to the current value of the parameter θ— or subsampling MCMC akin to stochastic gradient Langevin) with connection with cut posteriors,

Then I skipped to the second workshop on Bayesian methods for distributional and semiparametric regression, to listen to my friend David Rossell’s talk on local variable selection. Which suffers more than in standard models under misspecification. Another talk involving cut posteriors, the cuts being on the spline bases…

The day and the workshop concluded with great talks by (my friends) Pierre Alquier and David Frazier. David centred his misspecification talk on cut posteriors. Managing to bring in shrinkage estimators (and mention Bill Strawderman!).

A wee stressful trip, since the races in Caen cancelled all buses and delayed the taxi enough to miss the train to Paris by 30s, catching the next available one leaving me less than one hour between the arrival of the train (delayed by construction work on the rail line) and boarding the flight at Charles de Gaulle airport, but fortunately the RER trains in Paris were running okay, there were no queues in the airport, and I thus made it in time with a bit of post-marathon jogging! (Only to be delayed at departure by one hour for stormy conditions over Germany and Austria). All this exercise proved helpful to sleep soundly and lengthily in the plane!

A passage to India & to 2025

Posted in Kids, pictures, Running, Travel with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on January 16, 2025 by xi'an

Another great trip to India in association with the bi-yearly Winter IISA (International Indian Statistical Association) conference, including the opportunity to visit Kerala since the conference was taking place in Kochi (formerly Cochin). And a relatively smooth and relaxing trip as well! (And involuntarily repeating the Forsterite title of the earlier post.)

We left (as earlier) Paris on Xmath day, with de Gaulle airport busier than expected, including by the staff there, since a shortage of both border police officers and of luggage handlers caused a one-hour delay, with a full airplane to Bengaluru. Our flight took a rather circuitous route to avoid less secure air spaces, namely Ukraine, Israël, Syria, Iraq, and Iran, but also staying away from Houtis, hence turning sharply over the Sinaï to fly over Saudi Arabia. Massala chai onboard! My next seat neighbours sounded busy rescheduling their vacations (or others’?) to a visa-free country as some members of their party had apparently been denied boarding for that reason. (And I indeed had to frantically search through emails to recover the confirmation message from the Indian e-visa service provider at the boarding gate. But I had a great time reviewing grant proposals for most the flight!)

We were hoping for a smoother e-visa processing in Bengaluru than at our earlier visit for IISA 2022, and were indeed rewarded by no wait at all, leaving us more than five hours to switch between terminals for the leg to Kochi (and only too late realising too late there was an hotel inside the airport)! Single checked-in luggage was there at arrival in Kochi and prebooked taxi as well, to take sleep-deprived-us to a remote lodge in Mamalakkandam, at the foothill of the Western Ghats. Had some horrendous drives in this area, with small roads ill-equipped for crossing tourist buses, requiring several minutes and people directing vehicle choreographies from the pavement at times.Very relaxing stay, with tolerable temperatures, although we missed the elephants if not their multiple and fresh tracks and droppings! And managed to see a couple of Sri Lanka frogmouths, who really look like ChatGPT rendering of a chimera between a frog and a starling! And visited a small rubber farm thanks to the half-taxi-half-farming owner (with cocoa, cardamom and pepper parting gifts!).

This passage was also a foodie’s dream trip, as we discovered many new dishes, like a spicy chicken cooked in bamboo stick in an open fire and many local breads, e.g., illyappams, parathas, idlis, vellayapams, puttus, challas, and orottis, plus plenty of local dishes. Bought all kinds of spices. Celebrated New Year’s eve in Kochi by a light dinner with friends, at a local Saravana Bhavan branch that actually differed much from the one around the corner from Gare du Nord, Paris! I could not recognise most items in the menu, besides their landmark dosas and chaats, and some orders proved quite unexpected if pleasant surprises (albeit too spicy for my companions!) and was awaken a wee bit later by fireworks all over Kochi that seemed to get on forever. I could not reach a tea plantation near Munnar and further found it impossible to buy local tea, until reaching Kochi where some shops had green teas of unclear origin. It proved equally impossible to spot Munnar tea sold by reliable online tea companies.

The last station was Kuramakom, on the shores of the huge Vembanad Lake—longer than the Finger lakes surrounding Ithaca, New York—, where we saw plenty of birds as we were ferried through the backwaters at dusk. Visited the local bird sanctuary, at dawn to optimize our chances, but they remained low enough to only see a … tortoise crossing the path and a large fruit bat flying away. Day temperatures were such as making reading in the shade almost compulsory, which helped cleaning the pile of books I had taken with me.

The trip back was uneventful and reasonably delayed, considering that Delhi airport was shrouded in heavy fog and pollution, and as cold as Paris. With the incredible coïncidence of Didier Fraix-Burnet (IPAG) travelling the same flight Delhi-Paris as me, him returning from visiting Asis Chattopadhyay at the University of Calcutta—whom I visited over New Year’s Eve almost a decade ago And made it back to Paris, with our bags of groceries, with a -23⁰ temperature drop, instead of the -38⁰ drop we would have experienced a day earlier.

travel woes

Posted in Travel, University life with tags , , , , , , , , , , , , , , , , on April 29, 2024 by xi'an

On my last trip to Warwick, the local (RER) train I boarded broke on its way to the CDG airport, after hitting something in a tunnel just three stops short of the airport, with so much delay and misleading communication that I missed my flight. While a minor issue for me, since I managed to work (and blog) in an airport lounge for most of the day—where I crossed path with Numerobis—, while waiting for the only flight to B’ham, this made me to reflect anew on the very poor state of the transportation network in Paris and its suburbs, with such incidents (power failures, broken rails, vetust engines, stolen cables, idiots on the tracks, &tc., even without mentioning the strikes) more and more the norm. And to wonder at how the ancient and bursting network is going to cope with the incoming flow of visitors attending the Olympics this summer… Actually, when compared with the other cities with a fairly reasonably efficient airport connection I experienced, it remains a mystery to me why the Greater Paris conurbation—whose president, Valérie Pécresse, was apparently deemed the main culprit for our train break by an incensed fellow passenger that morning!—has kept for years postponing the construction of a dedicated rail line between the airport and central Paris, as the unpredictable and uncomfortable suburban train is not delivering the intended message to Paris visitors and their still-growing contribution to the French GNP. But with the growing public opposition to any new infrastructure, incl. trains, this is unlikely to happen!