Archive for cycle path

EM degeneracy

Posted in pictures, Statistics, Travel, University life with tags , , , , , , , , , , , , , , , , , , , , , on June 16, 2021 by xi'an

At the MHC 2021 conference today (to which I biked to attend for real!, first time since BayesComp!) I listened to Christophe Biernacki exposing the dangers of EM applied to mixtures in the presence of missing data, namely that the algorithm has a rising probability to reach a degenerate solution, namely a single observation component. Rising in the proportion of missing data. This is not hugely surprising as there is a real (global) mode at this solution. If one observation components are prohibited, they should not be accepted in the EM update. Just as in Bayesian analyses with improper priors, the likelihood should bar single or double  observations components… Which of course makes EM harder to implement. Or not?! MCEM, SEM and Gibbs are obviously straightforward to modify in this case.

Judith Rousseau also gave a fascinating talk on the properties of non-parametric mixtures, from a surprisingly light set of conditions for identifiability to posterior consistency . With an interesting use of several priors simultaneously that is a particular case of the cut models. Namely a correct joint distribution that cannot be a posterior, although this does not impact simulation issues. And a nice trick turning a hidden Markov chain into a fully finite hidden Markov chain as it is sufficient to recover a Bernstein von Mises asymptotic. If inefficient. Sylvain LeCorff presented a pseudo-marginal sequential sampler for smoothing, when the transition densities are replaced by unbiased estimators. With connection with approximate Bayesian computation smoothing. This proves harder than I first imagined because of the backward-sampling operations…

a rush to grade

Posted in Kids, pictures, Statistics, University life with tags , , , , , , , , , , on October 29, 2020 by xi'an

to bike or not to bike

Posted in Kids, pictures, Running, Travel with tags , , , , , , , , , on March 22, 2020 by xi'an

A recent debate between the candidates to the Paris mayorship, including a former Health minister and physician, led to arguments as to whether or not biking in Paris is healthy. Obviously, it is beneficial for the community, but the question is rather about the personal benefits vs dangers of riding a bike daily to work. Extra physical activity on the one hand, exposition to air pollution and accidents on the other hand. With an accident rate that increased during the recent strikes, but at a lesser rate (153%) than the number of cyclists in the streets of Paris (260%). While I do not find the air particularly stinky or unpleasant on my daily 25km, except in the frequent jams between Porte d’Auteuil and Porte de la Muette, and while I haven’t noticed a direct impact on my breathing or general shape, I try to avoid rush hours, especially on the way back home with a good climb near Porte de Versailles (the more on days when it is jammed solid with delivery trucks for the nearby exhibition centre). As for accidents, trying to maintain constant vigilance and predicting potential fishtails is the rule, as is avoiding most bike paths as I find them much more accident-prone than main streets… (Green lights are also more dangerous than red lights, in my opinion!) Presumably, so far at least, benefits outweight the costs!

local mayhem, again and again and again…

Posted in Kids, pictures, Travel, University life with tags , , , , , , , , , , , , , , , , , , on December 27, 2019 by xi'an

The public transports in France and in particular in Paris have now been on strike for three weeks. In connection with a planned reform of the retirement conditions of workers with special status, like those in the train and metro companies, who can retire earlier than the legal age (62). As usual with social unrest in France, other categories joined the strike and the protest, including teachers and health service public workers, as well as police officers, fire-fighters and opera dancers, and even some students. Below are some figures from the OECD about average retirement conditions in nearby EU countries that show that these conditions are apparently better in France. (With the usual provision that these figures have been correctly reported.) In particular, the life expectancy at the start of retirement is the highest for both men and women. Coincidence (or not), my UCU affiliated colleagues in Warwick were also on strike a few weeks ago about their pensions…

Travelling through and around Paris by bike, I have not been directly affected by the strikes (as heavy traffic makes biking easier!), except for the morning of last week when I was teaching at ENSAE, when I blew up a tyre midway there and had to hop to the nearest train station to board the last train of the morning, arriving (only) 10mn late. Going back home was only feasible by taxi, which happened to be large enough to take my bicycle as well… Travelling to and from the airport for Vancouver and Birmingham was equally impossible by public transportation, meaning spending fair amounts of time in and money on taxis! And listening to taxi-drivers’ opinions or musical tastes. Nothing to moan about when considering the five to six hours spent by some friends of mine to get to work and back.

free Fall

Posted in Statistics with tags , , , , , , , on September 15, 2018 by xi'an

Summer is off, Fall is back, as now the open air swimming pool has closed, the park opens too late for my schedule, shorts have all but disappeared from the streets and classrooms, the last raspberries have dried out, green tomatoes in my garden are unlikely to get any redder, the ant traps are no longer needed, the watering hose has been stored inside along the umbrella, and eating outdoors becomes a challenge, but the return from the traditional August vacation break has seen an explosion in the number of cyclists on my way to Dauphine, meaning I end up on most trips biking with (or against) other [100% nuke free] cyclists, with an improved trip duration (if not safer trips!). And no connection with the free fall tee!