don’t wear your helmet, you could have a bike accident!

As once in a while reappears the argument that wearing a bike helmet increases one’s chances of a bike accident. In the current case, it is to argue against a French regulation proposal that helmets should be compulsory for all cyclists. Without getting now into the pros and cons of compulsory helmet laws (enforced in Argentina, Australia, and New Zealand, as well as some provinces of Canada), I see little worth in the study cited by Le Monde towards this argument. As the data is poor and poorly analysed. First, there is a significant fraction of cycling accidents when the presence of an helmet is unknown. Second, the fraction of cyclists wearing helmets is based on a yearly survey involving 500 persons in a few major French cities. The conclusion that there are three times more accidents among cyclists wearing helmets than among cyclists not wearing helmets is thus not particularly reliable. Rather than the highly debatable arguments that (a) seeing a cyclist with an helmet would reduce the caution of car or bus drivers, (b) wearing an helmet would reduce the risk aversion of a cyclist, (c) sport-cyclists are mostly wearing helmets but their bikes are not appropriate for cities (!), I would not eliminate [as the authors do] the basic argument that helmeted cyclists are on average traveling longer distances. With a probability of an accident that necessarily  increases with the distance traveled. While people renting on-the-go bikes are usually biking short distances and almost never wear helmets. (For the record, I mostly wear a [bright orange] helmet but sometimes do not when going to the nearby bakery or swimming pool… Each time I had a fall, crash or accident with a car, I was wearing an helmet and I once hit my head or rather the helmet on the ground, with no consequence I am aware of!)

ABC in Svalbard [the day after]

The following and very kind email was sent to me the day after the workshop

thanks once again to make the conference possible. It was full of interesting studies within a friendly environment, I really enjoyed it. I think it is not easy to make a comfortable and inspiring conference in a remote version and across two continents, but this has been the result. I hope to be in presence (maybe in Svalbard!) the next edition.

and I fully agree to the talks behind full of interest and diverse. And to the scheduling of the talks across antipodal locations a wee bit of a challenge, mostly because of the daylight saving time  switches! And to seeing people together being a comfort (esp. since some were enjoying wine and cheese!).

I nonetheless found the experience somewhat daunting, only alleviated by sharing a room with a few others in Dauphine and having the opportunity to react immediately (and off-the-record) to the on-going talk. As a result I find myself getting rather scared by the prospect of the incoming ISBA 2021 World meeting. With parallel sessions and an extensive schedule from 5:30am till 9:30pm (in EDT time, i.e. GMT-4) that nicely accommodates the time zones of all speakers. I am thus thinking of (safely) organising a local cluster to attend the conference together and recover some of the social interactions that are such an essential component of [real] conferences, including students’ participation. It will of course depend on whether conference centres like CIRM reopen before the end of June. And if enough people see some appeal in this endeavour. In the meanwhile, remember to register for ISBA 2021 and for free!, before 01 May.

likelihood-free and summary-free?

My friends and coauthors Chris Drovandi and David Frazier have recently arXived a paper entitled A comparison of likelihood-free methods with and without summary statistics. In which they indeed compare these two perspectives on approximate Bayesian methods like ABC and Bayesian synthetic likelihoods.

“A criticism of summary statistic based approaches is that their choice is often ad hoc and there will generally be an  inherent loss of information.”

In ABC methods, the recourse to a summary statistic is often advocated as a “necessary evil” against the greater evil of the curse of dimension, paradoxically providing a faster convergence of the ABC approximation (Fearnhead & Liu, 2018). The authors propose a somewhat generic selection of summary statistics based on [my undergrad mentors!] Gouriéroux’s and Monfort’s indirect inference, using a mixture of Gaussians as their auxiliary model. Summary-free solutions, as in our Wasserstein papers, rely on distances between distributions, hence are functional distances, that can be seen as dimension-free as well (or criticised as infinite dimensional). Chris and David consider energy distances (which sound very much like standard distances, except for averaging over all permutations), maximum mean discrepancy as in Gretton et al. (2012), Cramèr-von Mises distances, and Kullback-Leibler divergences estimated via one-nearest-neighbour formulas, for a univariate sample. I am not aware of any degree of theoretical exploration of these functional approaches towards the precise speed of convergence of the ABC approximation…

“We found that at least one of the full data approaches was competitive with or outperforms ABC with summary statistics across all examples.”

The main part of the paper, besides a survey of the existing solutions, is to compare the performances of these over a few chosen (univariate) examples, with the exact posterior as the golden standard. In the g & k model, the Pima Indian benchmark of ABC studies!, Cramèr does somewhat better. While it does much worse in an M/G/1 example (where Wasserstein does better, and similarly for a stereological extremes example of Bortot et al., 2007). An ordering inversed again for a toad movement model I had not seen before. While the usual provision applies, namely that this is a simulation study on unidimensional data and a small number of parameters, the design of the four comparison experiments is very careful, eliminating versions that are either too costly or too divergence, although this could be potentially criticised for being unrealistic (i.e., when the true posterior is unknown). The computing time is roughly the same across methods, which essentially remove the call to kernel based approximations of the likelihood. Another point of interest is that the distance methods are significantly impacted by transforms on the data, which should not be so for intrinsic distances! Demonstrating the distances are not intrinsic…

ABC not in Svalbard [this time!]

Alas, thrice alas!, there will be no one attending the ABC in Svalbard in Svalbard  next April. As the travel conditions to and around Norway are getting tougher, it is just too unrealistic to expect traveling to the Far North even from Oslo. Too bad, but hopefully there will be another opportunity in a near enough future…

However, don’t give up the fight!, the mirror meetings in Brisbane and Grenoble are still planned to take place, along with an on-line version accommodating most of the speakers invited so far. Anyone interested in holding another mirror meeting?! Please contact me.

ABC in Svalbard [update]

Even though no one can tell at this stage who will be allowed to travel to Svalbard mid April 2021, we are keeping the workshop to physically take place as planned in Longyearbyen. With at least a group of volunteers made of researchers from Oslo (since at the current time, travel between mainland Norway and Svalbard is authorised). The conference room reservation has been confirmed yesterday and there are a few hotel rooms pre-booked through Hurtigrutensvlabard.com. Anyone planning to attend just need to (i) register on the workshop webpage, (ii) book an hotel room for the duration of the workshop (or more)., and (iii) reserve a plane ticket as there are not that many flights planned.

Obviously this option should only attract a few brave souls (from nearby countries). We are thus running at the same time three mirror workshops in Brisbane (QUT), Coventry (University of Warwick), and Grenoble (IMAG & INRIA). Except for Warwick, where the current pandemic restrictions do not allow for a workshop to take place, the mirror workshops will take place in university buildings and be face-to-face (with video connections as well). Julyan Arbel has set-up a mirror webpage as well. With a (free) registration deadline of 31 March, the workshop being open to all who can attend. Hopefully enough of us will gather here or there to keep up with the spirit of the earlier ABC workshops. (To make the mirror places truly ABCesque, it should have been set in A as Autrans rather than Grenoble!)