Archive for Sydney

Scott Sisson’s ABC seminar in Paris [All about that Bayes]

Posted in pictures, Statistics, Travel, University life with tags , , , , , , , , , , , on January 20, 2020 by xi'an

On the “All about that Bayes” seminar tomorrow (Tuesday 21 at 3p.m., room 42, AgroParisTech, 16 rue Claude Bernard, Paris 5ième), Scott Sisson, School of Mathematics and Statistics at UNSW, and visiting Paris-Dauphine this month, will give a talk on

Approximate posteriors and data for Bayesian inference

Abstract
For various reasons, including large datasets and complex models, approximate inference is becoming increasingly common. In this talk I will provide three vignettes of recent work. These cover a) approximate Bayesian computation for Gaussian process density estimation, b) likelihood-free Gibbs sampling, and c) MCMC for approximate (rounded) data.

Australia’s burning and it ain’t gonna stop anytime soon…

Posted in pictures with tags , , , , , , on January 2, 2020 by xi'an

Australian theocracy

Posted in Kids, pictures, Travel with tags , , , , on December 25, 2019 by xi'an

Examples from The Guardian at which discrimination based on religious arguments should become legal in Australia:

  • A doctor may tell a transgender patient of their religious belief that God made men and women in his image and that gender is therefore binary (EM)

  • A single mother who, when dropping her child off at daycare, may be told by a worker that she is sinful for denying her child a father (Public Interest Advocacy Centre)

  • A woman may be told by a manager that women should submit to their husbands or that women should not be employed outside the home (PIAC)

  • A student with disability may be told by a teacher their disability is a trial imposed by God (PIAC)

  • A person of a minority faith may be told by a retail assistant from another religion that they are a “heathen destined for eternal damnation” (PIAC).

  • A Catholic doctor refusing to provide contraception to all patients (EM) or to prescribe hormone treatment for gender transition (Equality Australia, Just Equal, LGBTI Health Alliance)

  • A Catholic nurse who refused to participate in abortion procedures (EM) or to provide the morning-after pill to a woman admitted to hospital after a sexual assault (Equality Australia)

  • A pharmacist refusing to provide the pill to women for contraceptive use (EM), or hormone treatment (Public Interest Advocacy Centre, LGBTI Health Alliance)

  • A doctor could refuse to prescribe post-exposure prophylaxis (PEP) within the required 72-hour window to a patient whose condom broke during a sexual encounter on the basis of religious beliefs that forbid sexual activity outside of marriage (Equality Australia)

  • A psychiatrist could say to a woman with depression that “she should be looking forward to the kingdom of heaven”. (Equality Australia)

  • A Jewish school may require that its staff and students be Jewish and accordingly refuse to hire or admit someone because they were not Jewish (EM)

  • A student attends the same religious school through their primary and secondary education. At 16 they lose faith in the religion of the school and tell a teacher that they are now agnostic. The school would be able to expel, suspend or otherwise punish, for example, give detention to the student (PIAC)

  • A homeowner seeking a tenant for their spare room may require that the tenant be of the same religious belief or activity as the homeowner (EM).

common derivation for Metropolis–Hastings and other MCMC algorithms

Posted in Books, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , , on July 25, 2016 by xi'an

Khoa Tran and Robert Kohn from UNSW just arXived a paper on a comprehensive derivation of a large range of MCMC algorithms, beyond Metropolis-Hastings. The idea is to decompose the MCMC move into

  1. a random completion of the current value θ into V;
  2. a deterministic move T from (θ,V) to (ξ,W), where only ξ matters.

If this sounds like a new version of Peter Green’s completion at the core of his 1995 RJMCMC algorithm, it is bedowntown Sydney from under Sydney Harbour bridge, July 15, 2012cause it is indeed essentially the same notion. The resort to this completion allows for a standard form of the Metropolis-Hastings algorithm, which leads to the correct stationary distribution if T is self-inverse. This representation covers Metropolis-Hastings algorithms, Gibbs sampling, Metropolis-within-Gibbs and auxiliary variables methods, slice sampling, recursive proposals, directional sampling, Langevin and Hamiltonian Monte Carlo, NUTS sampling, pseudo-marginal Metropolis-Hastings algorithms, and pseudo-marginal Hamiltonian  Monte Carlo, as discussed by the authors. Given this representation of the Markov chain through a random transform, I wonder if Peter Glynn’s trick mentioned in the previous post on retrospective Monte Carlo applies in this generic setting (as it could considerably improve convergence…)

ABC in Sydney [guest post #2]

Posted in pictures, Statistics, University life with tags , , , on July 24, 2014 by xi'an

[Here is a second guest post on the ABC in Sydney workshop, written by Chris Drovandi]

First up Dennis Prangle presented his recent work on “Lazy ABC”, which can speed up ABC by potentially abandoning model simulations early that do not look promising. Dennis introduces a continuation probability to ensure that the target distribution of the approach is still the ABC target of interest. In effect, the ABC likelihood is estimated to be 0 if early stopping is performed otherwise the usual ABC likelihood is inflated by dividing by the continuation probability, ensuring an unbiased estimator of the ABC likelihood. The drawback is that the ESS (Dennis uses importance sampling) of the lazy approach will likely be less than usual ABC for a fixed number of simulations; but this should be offset by the reduction in time required to perform said simulations. Dennis also presented some theoretical work for optimally tuning the method, which I need more time to digest.
This was followed by my talk on Bayesian indirect inference methods that use a parametric auxiliary model (a slightly older version here). This paper has just been accepted by Statistical Science.
Morning tea was followed by my PhD student, Brenda Vo, who presented an interesting application of ABC to cell spreading experiments. Here an estimate of the diameter of the cell population was used as a summary statistic. It was noted after Brenda’s talk that this application might be a good candidate for Dennis’ Lazy ABC idea. This talk was followed by a much more theoretical presentation by Pierre del Moral on how particle filter methodologies can be adapted to the ABC setting and also a general framework for particle methods.
Following lunch, Guilherme Rodrigues presented a hierarchical Gaussian Process model for kernel density estimation in the presence of different subgroups. Unfortunately my (lack of) knowledge on non-parametric methods prevents me from making any further comment except that the model looked very interesting and ABC seemed a good candidate for calibrating the model. I look forward to the paper appearing on-line.
The next presentation was by Gael Martin who spoke about her research on using ABC for estimation of complex state space models. This was probably my favourite talk of the day, and not only because it is very close to my research interests. Here the score of the Euler discretised approximation of the generative model was used as summary statistics for ABC. From what I could gather, it was demonstrated that the ABC posterior based on the score or the MLE of the auxiliary model were the same in the limit as ε 0 (unless I have mis-interpreted). This is a very useful result in itself; using the score to avoid an optimisation required for the MLE can save a lot of computation. The improved approximations of the proposed approach compared with the results that use the likelihood of the Euler discretisation were quite promising. I am certainly looking forward to this paper coming out.
Matt Moores drew the short straw and had the final presentation on the Friday afternoon. Matt spoke about this paper (an older version is available here), of which I am now a co-author. Matt’s idea is that doing some pre-simulations across the prior space and determining a mapping between the parameter of interest and the mean and variance of the summary statistic can significantly speed up ABC for the Potts model, and potentially other ABC applications. The results of the pre-computation step are used in the main ABC algorithm, which no longer requires simulation of pseudo-data but rather a summary statistic can be simulated from the fitted auxiliary model in the pre-processing step. Whilst this approach does introduce a couple more layers of approximation, the gain in computation time was up to two orders of magnitude. The talks by Matt, Gael and myself gave a real indirect inference flavour to this year’s ABC in…

ABC in Sydney [guest post]

Posted in pictures, Statistics, University life with tags , , , on July 18, 2014 by xi'an

[Scott Sisson sent me this summary of the ABC in Sydney meeting that took place two weeks ago.]

ABC speakersFollowing on from ABC in Paris (2009), ABC in London (2011) and ABC in Rome (2013), the fourth instalment of the international workshops in Approximate Bayesian Computation (ABC) was held at UNSW in Sydney on 3rd-4th July 2014. The first antipodean workshop was held as a satellite to the huge (>550 registrations) IMS-ASC-2014 International Conference, also held in Sydney the following week.

ABC in Sydney was created in two parts. The first, on the Thursday, was held as an “introduction to ABC” for people who were interested to find out more about the subject, but who had not particularly been exposed to the area before. Rather than have a single brave individual give the introductory course over several hours, the expository presentation was “crowdsourced” from several experienced researchers in the field, with each being given 30 minutes to present on a particular aspect of ABC. In this way, Matthew Moores (QUT), Dennis Prangle (Reading), Chris Drovandi (QUT), Zach Aandahl (UNSW) and Scott Sisson (UNSW) covered the ABC basics over the course of 6 presentations and 3 hours.

The second part of the workshop, on Friday, was the more usual collection of research oriented talks. In the morning session, Dennis Prangle spoke about “lazy ABC,” a method of stopping the generation of computationally demanding dataset simulations early, and Chris Drovandi discussed theoretical and practical aspects of Bayesian indirect inference. This was followed by Brenda Nho Vo (QUT) presenting an application of ABC in stochastic cell spreading models, and by Pierre Del Moral (UNSW) who demonstrated many theoretical aspects of ABC in interacting particle systems. After lunch Guilherme Rodrigues (UNSW) proposed using ABC for Gaussian process density estimation (and introduced the infinite-dimensional functional regression adjustment), and Gael Martin (Monash) spoke on the issues involved in applying ABC to state space models. The final talk of the day was given by Matthew Moores who discussed how online ABC dataset generation could be circumvented by pre-computation for particular classes of models.

In all, over 100 people registered for and attended the workshop, making it an outstanding success. Of course, this was helped by the association with the following large conference, and the pricing scheme — completely free! — following the tradition of the previous workshops. Morning and afternoon teas, described as “the best workshop food ever!” by several attendees, was paid for by the workshop sponsors: the Bayesian Section of the Statistical Society of Australia, and the ARC Centre of Excellence in Mathematical and Statistical Frontiers.

Here’s looking forward to the next workshop in the series!

Flaggermusmannen [book review]

Posted in Books, Kids, Travel with tags , , , , , , on May 25, 2014 by xi'an

“Cold, concise statistics. Keyword number one is statistical significance. In other words, we are looking for a system that cannot be explained by statistical chance (…) this group constitutes less than five percent of the female population. Yet I was left with seven murders and over forty rapes.” J. Nesbø

Another first novel! The Bat (Flaggermusmannen) by  Jo Nesbø has been sitting in my bedside book pile for quite a while, until I decided to read it a few days ago. It is the first appearance of Inspector Harry Hole in a published book and was written in 1997, although translated into English much much later. (The book was nominated as Best Norwegian Crime Novel of the Year and as Best Nordic Crime Novel of the Year.)

“Life consists of a series of quite improbable chance occurrences (…) What bothers me is that I’ve got that lottery number too many times in a row.” J. Nesbø

I read the (later) novel The Redeemer a few years ago, taking place mostly in Norway and kept a globally positive impression about the book, even though the plot was a bit stretched… The Bat has somewhat the same defects as The Ice Princess in that it sounds too much like an exercise in thriller writing, albeit in a much less clumsy style! The central character of Harry Hole is well-done, in an engaging-despite-his-shortcomings style and the way he gets along with most of the people he meets is rather realistic. However, the setting of the first novel in Australia (rather than Norway) is sort of a failure in that the country and Sydney are more caricatures than realistic in any degree…. For instance, every aboriginal Harry meets must resort to traditional tales involving emus and lizards and other local animals. One such tale would be ok but so many are just a bore. The title itself is connected to yet another aboriginal myth. And to the murders occurring way too often in the novel. Similarly, every foreign backpacker met in the pages of The Bat is either dumb or on her way to become a waitress to recover from a failed love affair. And a major character is a transvestite playing in a theatre, maybe because Nesbø has watched Priscilla Queen of the Desert a few years earlier… And the Australian police officers sound both very heavy in colloquialism and quite light in detective skills. Lacking an obvious connection to a series of young women murders throughout Australia. The second part of the novel gets too artificial to remain gripping and I completed the book with a feeling of chore accomplished…, not of surprise or shock at the resolution of the murders. I thus concur with many other readers of the book that it is certainly far from being the best in the series!