## hands-on probability 101

Posted in Books, Kids, pictures, Statistics, University life with tags , , , , , , , , , on April 3, 2021 by xi'an

When solving a rather simple probability question on X validated, namely the joint uniformity of the pair

$(X,Y)=(A-B+\mathbb I_{A

when A,B,C are iid U(0,1), I chose a rather pedestrian way and derived the joint distribution of (A-B,C-B), which turns to be made of 8 components over the (-1,1)² domain. And to conclude at the uniformity of the above, I added a hand-made picture to explain why the coverage by (X,Y) of any (red) square within (0,1)² was uniform by virtue of the symmetry between the coverage by (A-B,C-B) of four copies of the (red) square, using color tabs that were sitting on my desk..! It did not seem to convince the originator of the question, who kept answering with more questions—or worse an ever-changing question, reproduced in real time on math.stackexchange!, revealing there that said originator was tutoring an undergrad student!—but this was a light moment in a dreary final day before a new lockdown.

## a journal of the plague year [are we there yet?!]

Posted in Books, Kids, Mountains, pictures, Travel with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on March 20, 2021 by xi'an

Read the next volume of the Witcher series, Baptism of Fire, with even less enthusiasm than for the previous one, as the momentum of the series seems to have stalled… (Despite reading some highly positive reviews.) Some dialogues are funny enough, along with progressive views not particularly common in fantasy, like the support of reproductive rights, incl. abortion (and even less supported in the home country of the author, Andrzej Sapkowski!). But overall, not much happening and too much infodump!

Baked Ethiopian lentils & spinach mix, to get along with a slow cooking Ethiopian beef stew. And cooked more Venetian dishes. And had a great Korean streetfood dinner at (or from) MamiBaba by Quinsou, near Montparnasse, with pajeon (the cousin to okonomiyaki!) and kimchee. Accompanied by a first attempt at baking a chocolate pie.

Watched a few episodes of Alice in Borderland, vaguely suggested as hearsay by my daughter, but despite the fascinating scenes of an empty Tokyo, the plot is not particularly engaging, the tricks towards solving the game often lame, and the characters are not developed at all. Then watched Kurosawa’s Creepy, a gripping if not particularly realist psychological thriller that was premiered at the 2016 Berlin Film Festival. And reminded me of the much more disturbing Losey’s The Servant

Read two further volumes of John Harvey’s Charlie Resnick, in a random order, volumes that I found in and returned to the exchange section in front of our library as usual. And which I found almost as good as the first one, with its insistence on the humanity of each of the characters rather than indulging in manicheism. References to jazz pieces got a wee bit annoying by the third volume… And there is a maximal number of rye bread sandwiches with Polish pastrami I can swallow!

Watched also for the first time the fascinating The Wild Goose Lake (南方车站的聚会 which translates as A Rendez-Vous at a Station in the South), by Diao Yinan, a 2019 Cannes Festival selection, a psychological and violent noir film taking place in Wuhan among local gangs, when a gang boss kills by mistake a policeman after a very gory episode. The classical story line of the chase à la A bout de souffle is both tenuous and gripping, with an painful attention to colour and lightings, most scenes taking place at night with ghastly lights, with an intentional confusion between gangs of criminals and groups of cops, the final scene in full daylight making everything else sounding like a bad dream. The two main characters are striking, with an outlandish swan-like actress Gwei Lun-Mei. This also led me to watch the earlier Black Coal Thin Ice, which I also found impressive in terms of filming [that makes the cold and snow in this Northern city almost perceptible!] and definition of characters, once again involving Gwei Lun-Mei as the central, almost mute, and doomed, woman, but puzzling in terms of psychology and scenarios. (The shootout in the gallery is plain ridiculous imho.)

## on Astra and clots

Posted in Books, Kids, pictures, Statistics with tags , , , , , , , , , , , , on March 16, 2021 by xi'an

A tribune this morning in The Guardian by David Spiegelhalter on having no evidence that the Oxford/AstraZeneca vaccine causes blood clots.

“It’s a common human tendency to attribute a causal effect between different events, even when there isn’t one present: we wash the car and the next day a bird relieves itself all over the bonnet. Typical.”

David sets the 30 throboembolic events among the 5 million people vaccinated with AstraZeneca in perpective of the expected 100 deep vein thromboses a week within such a population. Which coincides with the UK’s Medicines and Healthcare Products Regulatory Agency statement that the blood clots are in par with the expected numbers in the vaccinated population. (The part of the tribune about the yellow card reports, based on 10 million vaccinated people, reiterates the remark but may prove confusing to some!) As for hoping for a rational approach to the issue,  … we would need a different type of vaccine, far from being available! As demonstrated by the decision to temporarily stop vaccinating with this vaccine, causing sure additional deaths in the coming weeks.

“Will we ever be able to resist the urge to find causal relationships between different events? One way of doing this would be promoting the scientific method and ensuring everyone understands this basic principle. Testing a hypothesis helps us see which hunches or assumptions are correct and which aren’t. In this way, randomised trials have proved the effectiveness of some Covid treatments and saved vast numbers of lives, while also showing us that some overblown claims about treatments for Covid-19, such as hydroxychloroquine and convalescent plasma, were incorrect.”

## probability that a vaccinated person is shielded from COVID-19?

Posted in Books, Statistics, Travel, University life with tags , , , , , , , , , , , , on March 10, 2021 by xi'an

Over my flight to Montpellier last week, I read an arXival on a Bayesian analysis of the vaccine efficiency. Whose full title is “What is the probability that a vaccinated person is shielded from Covid-19? A Bayesian MCMC based reanalysis of published data with emphasis on what should be reported as `efficacy'”, by Giulio D’Agostini and Alfredo Esposito. In short I was not particularly impressed.

“But the real point we wish to highlight, given the spread of distributions, is that we do not have enough data for drawing sound conclusion.”

The reason for this lack of enthusiasm on my side is that, while the authors’ criticism of an excessive precision in Pfizer, Moderna, or AstraZeneca press releases is appropriate, given the published confidence intervals are not claiming the same precision, a Bayesian reanalysis of the published outcome of their respective vaccine trial outcomes does not show much, simply because there is awfully little data, essentially two to four Binomial-like outcomes. Without further data, the modelling is one of a simple graph of Binomial observations, with two or three probability parameters, which results in a very standard Bayesian analysis that does depend on the modelling choices being made, from a highly unrealistic assumption of homogeneity throughout the population(s) tested for the vaccine(s), to a lack of hyperparameters that could have been shared between vaccinated populations. Parts of the arXival are unrelated and unnecessary, like the highly detailed MCMC algorithm for simulating the posterior (incl. JAGS code) to the reminiscence of Bayes’ and Laplace’s early rendering of inverse probability. (I find both interesting and revealing that arXiv, just like medRxiv, posts a warning on top of COVID related preprints.)

## race against COVID [mural]

Posted in Books, Kids, pictures, Running, Travel with tags , , , , , , , on March 7, 2021 by xi'an