## challenged books

Posted in Books, Kids with tags , , , , , , , , , on April 9, 2017 by xi'an

After reading that Margaret Atwood’s The Handmaid’s Tale was one of the most challenged books in the USA, where challenged means “documented requests to remove materials from school or libraries”, I went to check on the website of the American Library Association for other titles, and found that The Curious Incident of the Dog in the Nigh-time and the Bible made it to the top 10 in 2015, with Of Mice and Men, Harry Potter, The Adventures of Huckleberry Finn, Brave New World, Hunger Games, Slaughterhouse Five, Cal, several of Roald Dahl’s and of Toni Morrisson’s books, Persepolis, and Tintin in America [and numerous others] appearing in the list… (As read in several comments, it is quite a surprise Shakespeare is not part of it!)

What is most frightening about those challenges and calls for censorship is that a growing portion of the reasons given against the books is “diversity“, namely that they propose a different view point, were it religious (or atheist), gender-related, ethnic, political, or disability-related.

## the maths of Jeffreys-Lindley paradox

Posted in Books, Kids, Statistics with tags , , , , , , , on March 26, 2015 by xi'an

Cristiano Villa and Stephen Walker arXived on last Friday a paper entitled On the mathematics of the Jeffreys-Lindley paradox. Following the philosophical papers of last year, by Ari Spanos, Jan Sprenger, Guillaume Rochefort-Maranda, and myself, this provides a more statistical view on the paradox. Or “paradox”… Even though I strongly disagree with the conclusion, namely that a finite (prior) variance σ² should be used in the Gaussian prior. And fall back on classical Type I and Type II errors. So, in that sense, the authors avoid the Jeffreys-Lindley paradox altogether!

The argument against considering a limiting value for the posterior probability is that it converges to 0, 21, or an intermediate value. In the first two cases it is useless. In the medium case. achieved when the prior probability of the null and alternative hypotheses depend on variance σ². While I do not want to argue in favour of my 1993 solution

$\rho(\sigma) = 1\big/ 1+\sqrt{2\pi}\sigma$

since it is ill-defined in measure theoretic terms, I do not buy the coherence argument that, since this prior probability converges to zero when σ² goes to infinity, the posterior probability should also go to zero. In the limit, probabilistic reasoning fails since the prior under the alternative is a measure not a probability distribution… We should thus abstain from over-interpreting improper priors. (A sin sometimes committed by Jeffreys himself in his book!)

## self-healing umbrella sampling

Posted in Kids, pictures, Statistics, University life with tags , , , , , , , on November 5, 2014 by xi'an

Ten days ago, Gersende Fort, Benjamin Jourdain, Tony Lelièvre, and Gabriel Stoltz arXived a study about an adaptive umbrella sampler that can be re-interpreted as a Wang-Landau algorithm, if not the most efficient version of the latter. This reminded me very much of the workshop we had all together in Edinburgh last June. And even more of the focus of the molecular dynamics talks in this same ICMS workshop about accelerating the MCMC exploration of multimodal targets. The self-healing aspect of the sampler is to adapt to the multimodal structure thanks to a partition that defines a biased sampling scheme spending time in each set of the partition in a frequency proportional to weights. While the optimal weights are the weights of the sets against the target distribution (are they truly optimal?! I would have thought lifting low density regions, i.e., marshes, could improve the mixing of the chain for a given proposal), those are unknown and they need to be estimated by an adaptive scheme that makes staying in a given set the less desirable the more one has visited it. By increasing the inverse weight of a given set by a factor each time it is visited. Which sounds indeed like Wang-Landau. The plus side of the self-healing umbrella sampler is that it only depends on a scale γ (and on the partition). Besides converging to the right weights of course. The downside is that it does not reach the most efficient convergence, since the adaptivity weight decreases in 1/n rather than 1/√n.

Note that the paper contains a massive experimental side where the authors checked the impact of various parameters by Monte Carlo studies of estimators involving more than a billion iterations. Apparently repeated a large number of times.

The next step in adaptivity should be about the adaptive determination of the partition, hoping for a robustness against the dimension of the space. Which may be unreachable if I judge by the apparent deceleration of the method when the number of terms in the partition increases.

## Blake & Mortimer [volume 20]

Posted in Books, Kids with tags , , , , , on January 8, 2011 by xi'an

The second part of La Malédiction des Trente Deniers has now been out for a month. This is volume 20 in the Blake & Mortimer series, but the drawings are (predictably) done by yet another artist, Antoine Aubin! The style is not that bad (despite yet another awful cover!), but the scenario by van Hamme is very poor and the story does not move much when compared with the first volume: both heroes manage to find the location of the tomb of Judas and survive the trap set by their enemies… In the previous post about the first volume, I was mentioning the Indiana-Jonesque feeling about the plot, but it got much stronger with this volume! A ray of light comes from the heavens and burns the bad guy on the spot, while Blake & Mortimer escape by an unlikely underground river, along with the young Greek woman who had betrayed them to save her fiancé… The story borrows to earlier Blake & Mortimer stories, for instance the underground exploration is reminiscent of the beginning of l’Enigme de l’Atlantide, but the connection with Hergé s Tintin is stronger: the evil Beloukian behaves like Rastapopoulos in Flight 714, the shipwreck and the escape on a home-made raft is very similar to the one in The Red Sea Sharks, and several drawings evoke the atmosphere of The Black Island. Hopefully, the next volume will see a clear improvement by the new team, Sente and Julliard, but their botched Sarcophagi of the Sixth Continent is not very promising…