## day five at ISBA 22

Posted in Mountains, pictures, Running, Statistics, Travel, University life with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 4, 2022 by xi'an

Woke up even earlier today! Which left me time to work on switching to Leonard Cohen’s song titles for my slide frametitles this afternoon (last talk of the whole conference!), run once again to Mon(t) Royal as all pools are closed (Happy Canada Day!, except to “freedom convoy” antivaxxxers.) Which led to me meeting a raccoon by the side of the path (and moroons feeding wildlife).

Had an exciting time at the morning session, where Giacomo Zanella (formerly Warwick) talked on a mixture approach to leave-one-out predictives, with pseudo-harmonic mean representation, averaging inverse density across all observations. Better than harmonic? Some assumptions allow for finite variance, although I am missing the deep argument (in part due to Giacomo’s machine-gun delivery pace!) Then Alicia Corbella (Warwick) presented a promising entry into PDMP by proposing an automated zig-zag sampler. Pointing out on the side to Joris Bierkens’ webpage on the state-of-the-art PDMP methodology. In this approach, joint with with my other Warwick colleagues Simon Spencer and Gareth Roberts, the zig-zag sampler relies on automatic differentiation and sub-sampling and bound derivation, with “no further information on the target needed”. And finaly Chris Carmona presented a joint work with Geoff Nicholls that is merging merging cut posteriors and variational inference to create a meta posterior. Work and talk were motivated by a nice medieval linguistic problem where the latent variables impact the (convergence of the) MCMC algorithm [as in our k-nearest neighbour experience]. Interestingly using normalising [neural spline] flows. The pseudo-posterior seems to depend very much on their modularization rate η, which penalises how much one module influences the next one.

In the aft, I attended sort of by chance [due to a missing speaker in the copula session] to the end of a session on migration modelling, with a talk by Jason Hilton and Martin Hinsch focussing on the 2015’s mass exodus of Syrians through the Mediterranean,  away from the joint evils of al-Hassad and ISIS. As this was a tragedy whose modelling I had vainly tried to contribute to, I was obviously captivated and frustrated (leaning of the IOM missing migrant project!) Fitting the agent-based model was actually using ABC, and most particularly our ABC-PMC!!!

My own and final session had Gareth (Warwick) presenting his recent work with Jun Yang and Kryzs Łatuszyński (Warwick) on the stereoscopic projection improvement over regular MCMC, which involves turning the target into a distribution supported by an hypersphere and hence considering a distribution with compact support and higher efficiency. Kryzs had explained the principle while driving back from Gregynog two months ago. The idea is somewhat similar to our origaMCMC, which I presented at MCqMC 2016 in Stanford (and never completed), except our projection was inside a ball. Looking forward the adaptive version, in the making!

And to conclude this subjective journal from the ISBA conference, borrowing this title by (Westmount born) Leonard Cohen, “Hey, that’s not a way to say goodbye”… To paraphrase Bilbo Baggins, I have not interacted with at least half the participants half as much as I would have liked. But this was still a reunion, albeit in the new Normal. Hopefully, the conference will not have induced a massive COVID cluster on top of numerous scientific and social exchanges! The following days will tell. Congrats to the ISBA 2022 organisers for achieving a most successful event in these times of uncertainty. And looking forward the 2024 next edition in Ca’Foscari, Venezia!!!

## far south

Posted in Books, Statistics, Travel, University life with tags , , , , , , , , , , , , on February 23, 2022 by xi'an

## practical PDMP

Posted in Books, Statistics, University life with tags , , , , , , , , , , , on December 9, 2021 by xi'an

While in Warwick, last month, I attended a reading group on PDMPs where Filippo Pagani talked about practical PDMP, connected with a recent arXival by Bertazzi, Bierkens and Dobson. The central question when implementing PDMP is to find a realistic way of solving

$\int_0^\tau \lambda(x+tv,v)\text dt = \epsilon\quad\epsilon\sim\mathcal Exp(1)$

to decide on the stopping time (when the process ceases to be deterministic). The usual approach is to use Poisson thinning by finding an upper bound on λ, but this is either difficult or potentially inefficient (and sometimes both).

“finding a sharp bound M(s) [for Poisson thinning] can be an extremely challenging problem in most practical settings (…) In order to overcome this problem, we introduce discretisation schemes for PDMPs which make their
approximate simulation possible.”

Some of the solutions proposed in Bertazzi et al. are relying on

1. using a frozen (fixed) λ
2. discretising time and the integral (first order scheme)
3. allowing for more than a jump over a time interval (higher order schemes)
4. going through control variates (when gradient is Lipschitz and Hessian bounded, with known constants) as it produces a linear rate λ
5. subsampling (at least for Zig Zag)

with theoretical guarantees that the approximations are convergent, as the time step goes to zero. They (almost obviously) remain model dependent solutions (with illustrations for the Zig Zag and bouncy particle versions), with little worse case scenarios, but this is an extended investigation into making PDMPs more manageable!

## ISBA 2021.3

Posted in Kids, Mountains, pictures, Running, Statistics, Travel, University life, Wines with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 1, 2021 by xi'an

Now on the third day which again started early with a 100% local j-ISBA session. (After a group run to and around Mont Puget, my first real run since 2020!!!) With a second round of talks by junior researchers from master to postdoc level. Again well-attended. A talk about Bayesian non-parametric sequential taxinomy by Alessandro Zito used the BayesANT acronym, which reminded me of the new vave group Adam and the Ants I was listening to forty years ago, in case they need a song as well as a logo! (Note that BayesANT is also used for a robot using Bayesian optimisation!) And more generally a wide variety in the themes. Thanks to the j-organisers of this 100% live session!

The next session was on PDMPs, which I helped organise, with Manon Michel speaking from Marseille, exploiting the symmetry around the gradient, which is distribution-free! Then, remotely, Kengo Kamatani, speaking from Tokyo, who expanded the high-dimensional scaling limit to the Zig-Zag sampler, exhibiting an argument against small refreshment rates, and Murray Pollock, from Newcastle, who exposed quite clearly the working principles of the Restore algorithm, including why coupling from the past was available in this setting. A well-attended session despite the early hour (in the USA).

Another session of interest for me [which I attended by myself as everyone else was at lunch in CIRM!] was the contributed C16 on variational and scalable inference that included a talk on hierarchical Monte Carlo fusion (with my friends Gareth and Murray as co-authors), Darren’s call to adopt functional programming in order to save Bayesian computing from extinction, normalising flows for modularisation, and Dennis’ adversarial solutions for Bayesian design, avoiding the computation of the evidence.

Wes Johnson’s lecture was about stories with setting prior distributions based on experts’ opinions. Which reminded me of the short paper Kaniav Kamary and myself wrote about ten years ago, in response to a paper on the topic in the American Statistician. And could not understand the discrepancy between two Bayes factors based on Normal versus Cauchy priors, until I was told they were mistakenly used repeatedly.

Rushing out of dinner, I attended both the non-parametric session (live with Marta and Antonio!) and the high-dimension computational session on Bayesian model choice (mute!). A bit of a schizophrenic moment, but allowing to get a rough picture in both areas. At once. Including an adaptive MCMC scheme for selecting models by Jim Griffin. Which could be run directly over the model space. With my ever-going wondering at the meaning of neighbour models.

## general perspective on the Metropolis–Hastings kernel

Posted in Books, Statistics with tags , , , , , , , , , , , , , on January 14, 2021 by xi'an

[My Bristol friends and co-authors] Christophe Andrieu, and Anthony Lee, along with Sam Livingstone arXived a massive paper on 01 January on the Metropolis-Hastings kernel.

“Our aim is to develop a framework making establishing correctness of complex Markov chain Monte Carlo kernels a purely mechanical or algebraic exercise, while making communication of ideas simpler and unambiguous by allowing a stronger focus on essential features (…) This framework can also be used to validate kernels that do not satisfy detailed balance, i.e. which are not reversible, but a modified version thereof.”

A central notion in this highly general framework is, extending Tierney (1998), to see an MCMC kernel as a triplet involving a probability measure μ (on an extended space), an involution transform φ generalising the proposal step (i.e. þ²=id), and an associated acceptance probability ð. Then μ-reversibility occurs for

$\eth(\xi)\mu(\text{d}\xi)= \eth(\phi(\xi))\mu^{\phi}(\text{d}\xi)$

with the rhs involving the push-forward measure induced by μ and φ. And furthermore there is always a choice of an acceptance probability ð ensuring for this equality to happen. Interestingly, the new framework allows for mostly seamless handling of more complex versions of MCMC such as reversible jump and parallel tempering. But also non-reversible kernels, incl. for instance delayed rejection. And HMC, incl. NUTS. And pseudo-marginal, multiple-try, PDMPs, &c., &c. it is remarkable to see such a general theory emerging a this (late?) stage of the evolution of the field (and I will need more time and attention to understand its consequences).