## Gagner la guerre

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

Within a few sunny days of being consigned at home [by the “war” against the epidemics], I went through Jaworski’s Gagner la Guerre [To the victors go the spoils], which I had discovered in the list of the 101 favourite novels of Le Monde readers (or rather of whoever replied to the call since the survey was not restricted to Le Monde subscribers).  While I still have no clue how the book ended up at the 67th position in the list (!), next to Yourcenar’s fabulous L’Œuvre au Noir, I am still glad that this list pointed out the very existence of this book. Although not much more enlightened as to whom would include it in the “best novels ever”. (Warning: As the novel has not been translated from French into other languages, the review  below may be of limited appeal to most readers!)

A possible explanation for this paradox is that Jaworski is originally a creator of role-playing games and hence famous among some role-playing communities as well, who could have mobilized efficiently enough to bring him within the 101. The plot shows some influence of this role-playing expertise as the central character, a despicable, violent, sexist, xenophobic, rapist, murderous, anti-hero Benvenuto, moves from one danger to the next, while visiting the continent imagined by the author and meets characters from one fantasy race after the other: elves, dwarfs, near-orcs. Reminding me very much of the races in Warhammer, since fighting styles associated with each conveniently identified the different parts of the country. The home town of Benvenuto is a mix of Italian Renaissance state-cities, between Sienna and Venezia. Run by a Senate of rich families, fighting a Southern kingdom closely resembling the Ottoman empire, as in Guy Gavriel Kay’s Children of Earth and Sky. If in a much grittier style. It also reminded me of the fabulous Scott Lynch’s Gentlemen Bastards series, incl. Republic of Thieves. Sorcery is also involved here, whose role only appears progressively throughout the novel. Despite my usual annoyance at this choice, the writing style of the author, who also is a teacher of French literature in high school, always a first person narrative, ends up being a strength of the book, involving a rich multitude of language levels, from the vernacular to the antique, revealing as well a multitude of layers in Benvenuto (who finds himself anything but welcome from most places he visits!, including my living-room!!). None of them palatable however. To be perfectly clear, the book is an addictive page turner, despite an accumulation of details that sometimes delay the action, but which are nonetheless essential to make the book universe more substantial and complex. Highly recommended for French-speaking fans of grimdark pseudo-historical fantasy (over the legal age)!

## the grey bastards [book review]

Posted in Books with tags , , , , , , on July 21, 2019 by xi'an

Another almost random read, The Grey Bastards by Jonathan French is a light (if gritty) fantasy book that should appeal to Warhammer players. Including the use of hogs as mounts. In that the main characters are half-orcs, in a Universe where lots of species (also found in Warhammer) co-exist, if not peacefully. The idea of reverting the usual perspective on orcs as dumb killers was already found in Stan Nicholls’ Orcs, which I found better than the current Grey Bastards, especially because there is not much to distinguish these from humans, sentiments included, apart from their appearance, but this makes for an enjoyable travel read. Since the characters are rather well-drawn, the story is rather (too?) simple and one can see where it is heading. (Some reviews commented on the Tolkien-meets-Sons-of-Anarchy aspect of the book, but as I have not watched the series…) There is at least one central weakness to the plot that I will not reveal, which first comes as a great shocker but is then later explained by a rather lame arm bending blackmail, that makes the story not as strong as it could have been. Upon finishing the book I found out that (a) there was a second book in the series about to appear and (b) it has won the 2016 Self-published fantasy blog-off prize, a prize started by Mark Lawrence (author of Red Sister) to “shine a light on self-published fantasy” which sounds like a great idea, in that it helps the authors towards commercial publishing. The jury is made of 10  fantasy bloggers going through a rather time-consuming process.

## ABC for wargames

Posted in Books, Kids, pictures, Statistics with tags , , , , , , on February 10, 2016 by xi'an

I recently came across an ABC paper in PLoS ONE by Xavier Rubio-Campillo applying this simulation technique to the validation of some differential equation models linking force sizes and values for both sides. The dataset is made of battle casualties separated into four periods, from pike and musket to the American Civil War. The outcome is used to compute an ABC Bayes factor but it seems this computation is highly dependent on the tolerance threshold. With highly variable numerical values. The most favoured model includes some fatigue effect about the decreasing efficiency of armies along time. While the paper somehow reminded me of a most peculiar book, I have no idea on the depth of this analysis, namely on how relevant it is to model a battle through a two-dimensional system of differential equations, given the numerous factors involved in the matter…

## Posterior predictive p-values and the convex order

Posted in Books, Statistics, University life with tags , , , , , , , , , on December 22, 2014 by xi'an

Patrick Rubin-Delanchy and Daniel Lawson [of Warhammer fame!] recently arXived a paper we had discussed with Patrick when he visited Andrew and I last summer in Paris. The topic is the evaluation of the posterior predictive probability of a larger discrepancy between data and model

$\mathbb{P}\left( f(X|\theta)\ge f(x^\text{obs}|\theta) \,|\,x^\text{obs} \right)$

which acts like a Bayesian p-value of sorts. I discussed several times the reservations I have about this notion on this blog… Including running one experiment on the uniformity of the ppp while in Duke last year. One item of those reservations being that it evaluates the posterior probability of an event that does not exist a priori. Which is somewhat connected to the issue of using the data “twice”.

“A posterior predictive p-value has a transparent Bayesian interpretation.”

Another item that was suggested [to me] in the current paper is the difficulty in defining the posterior predictive (pp), for instance by including latent variables

$\mathbb{P}\left( f(X,Z|\theta)\ge f(x^\text{obs},Z^\text{obs}|\theta) \,|\,x^\text{obs} \right)\,,$

which reminds me of the multiple possible avatars of the BIC criterion. The question addressed by Rubin-Delanchy and Lawson is how far from the uniform distribution stands this pp when the model is correct. The main result of their paper is that any sub-uniform distribution can be expressed as a particular posterior predictive. The authors also exhibit the distribution that achieves the bound produced by Xiao-Li Meng, Namely that

$\mathbb{P}(P\le \alpha) \le 2\alpha$

where P is the above (top) probability. (Hence it is uniform up to a factor 2!) Obviously, the proximity with the upper bound only occurs in a limited number of cases that do not validate the overall use of the ppp. But this is certainly a nice piece of theoretical work.

## MCMSki IV [day 2.5]

Posted in Mountains, pictures, Statistics, University life with tags , , , , , , , , , on January 8, 2014 by xi'an

Despite a good rest during the ski break, my cold did not get away (no magic left in this world!) and I thus had a low attention span to attend the Bayesian statistics and Population genetics session: while Jukka Corander mentioned the improvement brought by our AMIS algorithm, I had difficulties getting the nature of the model, if only because he used a blackboard-like font that made math symbols too tiny to read. (Nice fonts, otherwise!), Daniel Lawson (of vomiting Warhammer fame!) talked about the alluring notion of a statistical emulator, and Barbara Engelhardt talked about variable selection in a SNP setting. I did not get a feeling on how handling ten millions of SNPs was possible in towards a variable selection goal.  My final session of the day was actually “my” invited session on ABC methods, where Richard Everitt presented a way of mixing exact approximation with ABC and synthetic likelihood (Wood, Nature) approximations. The resulting MAVIS algorithm is  not out yet. The second speaker was Ollie Ratman, who spoke on his accurate ABC that I have discussed many times here. And Jean-Michel Marin managed to drive from Montpelier, just in time to deliver his talk on our various explorations of the ABC model choice problem.

After a quick raclette at “home”, we headed back to the second poster session, where I had enough of a clear mind and not too much of a headache (!) to have several interesting discussions, incl. a new parallelisation suggested  by Ben Calderhead, the sticky Metropolis algorithm of Luca Martino, the airport management video of Jegar Pitchforth, the mixture of Dirichlet distributions for extremes by Anne Sabourin, not mentioning posters from Warwick or Paris. At the end of the evening  I walked back to my apartment with the Blossom skis we had brought in the morning to attract registrations for the ski race: not enough to make up for the amount charged by the ski school. Too bad, especially given Anto’s efforts to get this amazing sponsoring!