Archive for New Zealand

the luminaries [book review]

Posted in Books, Kids, Mountains, Travel with tags , , , , , , , , on April 18, 2015 by xi'an

I bought this book by Eleanor Catton on my trip to Pittsburgh and Toronto in 2013 (thanks to Amazon associates’ gains!), mostly by chance (and also because it was the most recent Man Booker Prize). After a few sleepless nights last week (when I should not have been suffering from New York jet lag!, given my sleeping pattern when abroad), I went through this rather intellectual and somewhat contrived mystery. To keep with tradition (!), the cover was puzzling me until I realised those were phases of the moon, in line with [spoiler!] the zodiacal underlying pattern of the novel, pattern I did not even try to follow for it sounded so artificial. And presumably restricted the flow of the story by imposing further constraints on the characters’ interactions.

The novel has redeeming features, even though I am rather bemused at it getting a Man Booker Prize. (When compared with, say, The Remains of the Day…) For one thing, while a gold rush story of the 1860’s, it takes place on the South Island of New Zealand instead of Klondike, around the Hokitika gold-field, on the West Coast, with mentions of places that brings memory of our summer (well, winter!) visit to Christchurch in 2006… The mix of cultures between English settlers, Maoris, and Chinese migrants, is well-documented and information, if rather heavy at times, bordering on the info-dump, and a central character like the Maori Te Rau Tauwhare sounds caricaturesque. The fact that the story takes place in Victorian times call Dickens to mind, but I find very little connection in either style or structure, nor with Victorian contemporaries like Wilkie Collins, and Victorian pastiches like Charles Palliser‘s Quincunx…. Nothing of the sanctimonious and moral elevation and subtle irony one could expect from a Victorian novel!

While a murder mystery, the plot is fairly upside down (or down under?!): the (spoiler!) assumed victim is missing for most of the novel, the (spoiler!) extracted gold is not apparently stolen but rather lacks owner(s), and the most moral character of the story ends up being the local prostitute. The central notion of the twelve men in a council each bringing a new light on the disappearance of Emery Staines is a neat if not that innovative literary trick but twelve is a large number which means following many threads, some being dead-ends, to gather an appearance of a view on the whole story. As in Rashomon, one finishes the story with a deep misgiving as to who did what, after so many incomplete and biased accountings. Unlike Rashomon, it alas takes forever to reach this point!

George’s dream

Posted in Kids, Travel with tags , , , , , on April 11, 2015 by xi'an

doublewashWhile I have shared this idea with many of my friends [in both senses that I mentioned it and that they shared the same feeling that it would be a great improvement], the first time I heard of the notion was in George Casella‘s kitchen in Ithaca, New York, in the early 1990’s… We were emptying the dishwasher together and George was reflecting that it would be so convenient to have a double dishwasher and remove the need to empty it altogether! Although, at the moral level, I think that we should do without dishwashers, I found this was a terrific idea and must have told the joke to most of my friends. I was nonetheless quite surprised and very pleased to receive the news from Nicole today that Fisher & Paykel (from Auckland, New Zealand) had gone all the way to produce a double dishwasher, or more exactly a double dishdrawer, perfectly suited to George’s wishes! (Pleased that she remembered the notion after all those years, not pleased with the prospect of buying a double dish washer for more than double the cost of [and a smaller volume than] a regular dishwasher!)

The Hobbit (once upon a very long time…)

Posted in Books, Kids, pictures with tags , , , , , , on December 25, 2014 by xi'an

“Will you follow me, one last time?”

With my daughter, we completed our Xmas Tolkien cycle by going together to see The battle of the five armies. As several have noted before me, the best thing I can say about this Hobbit series is that it is now… over! Just like the previous two instalments, watching Peter Jackson’s grand finale was mostly enjoyable, but mainly for the same reasons one enjoys visiting a venerable great-aunt once a year around Christmas, namely for bringing back memories of good times and shared laughs. Indeed, Jackson managed to link both sagas through his central character of Gandalf who, while overly fond of raised eyebrows and mischievous eyes, is certainly the most compelling character all over.  While the plot stretched too thinly to keep me enthralled, as I could not remember why the orcs and goblins were converging to Erebor at the same time as the elves and dwarves and men of Dale (unless it was to justify the future name of the battle?!), I soon got battle-weary of the repeated clashes between the various armies which sounded like straight copies from on-line war games and even more of the half-dozen duels, while the rescue of Gandalf from Dol Gurdur is unbearably clumsy, with an apocryphal appearance of the Nazguls. As too often in the story, the giant eagles were so instrumental to victory that one could only wonder why they had not been around from the start.

The comical parts are much sparser here than in the previous movies: hardly any screen time for Radagast’s rabbits, thank Sauron!, or for the jovial Dain with his great Scottish brogue and his war[t]hog opening, or yet for Thranduil’s moose to show its major advantage in battle, a few steps before being shot down, or for the war mountain goats who appeared then vanished at the moment of direst need, or for Bard to find a pre-historical skateboard. I also noted that the [dumb] orgs managed to invent a precursor of Chappe’s telegraph that alas could only transmit one symbol [since it was always taking the same shape!], that Legolas recreated the Matrix by walking on a disintegrating bridge, and that Thorin turned on gravity for a few crucial seconds in a movie where most characters seem to have no issue with falling, jumping or fighting without the slightest consideration for mechanics, with a strong tendency for characters to head-butt into walls…

“What this adaptation of “The Hobbit” can’t avoid by its final instalment is its predictability and hollow foundations.” NYT, Dec. 16, 2014

Other features I did not enjoy much: Thorin sulked way too long, Alferid outlasted its stay on screen by about 144 minutes, only to vanish unexpectedly, Bilbo seemed lost at the margins most of the movie, while the love story between Kili and Tauriel was really one addition too many to Tolkien’s book. The search for variety in the steeds of the various armies made me almost wish for more races on the battle-field as we could then have seen fighters on giant moles or on battle-hens… And everyone could have done without the “Dune moment”, with giant earth-worms breaking tunnels only to return to oblivion. Anyway, we have now been “There and Back Again” and can now settle in our own hobbit-hole to re-read the books and enjoy a certain nostalgia about the days where we could imagine on our own how Bilbo, Gandalf or Thorin would look like, while humming “Song of the Misty Mountains”…

Bayesian multimodel inference by RJMCMC: A Gibbs sampling approach

Posted in Books, pictures, Statistics, Travel, University life with tags , , , , , , on December 27, 2013 by xi'an

img_5288Barker (from the lovely city of Dunedin) and Link published a paper in the American Statistician last September that I missed, as I missed their earlier email about the paper since it arrived The Day After… The paper is about a new specification of RJMCMC, almost twenty years after Peter Green’s (1995) introduction of the method. The authors use the notion of a palette, “from which all model specific parameters can be calculated” (in a deterministic way). One can see the palette ψ as an intermediary step in the move between two models. This reduces the number of bijections, if not the construction of the dreaded Jacobians!, but forces the construction of pseudo-priors on the unessential parts of ψ for every model. Because the dimension of ψ is fixed, a Gibbs sampling interleaving model index and palette value is then implementable. The conditional of the model index given the palette is available provided there are not too many models under competitions, with the probabilities recyclable towards a Rao-Blackwell approximation of the model probability. I wonder at whether or not another Rao-Blackwellisation is possible, namely to draw from all the simulated palettes a sample for the parameter of an arbitrarily chosen model.

The Hobbit (a Smaug screen)

Posted in Books, Kids, Mountains, Travel with tags , , , , , , , , on December 25, 2013 by xi'an

In what seems to become a X’mas tradition, I went back to see a Hobbit movie with my kids. (If not in a Norman theatre, which permitted us to hear the original soundtrack.) And once more, we came out of the movie theatre with different reactions. Both my son and I thought it was better than the (very boring) first instalment. My daughter did not buy the dragon part (which is indeed difficult to buy!) and complained about the lack of depth and of this feeling of history and tradition that should come with elves. I completely agree with her analysis on this second part. The movie is too centred on action scenes—the park-ridesque escape from the Halls of Thranduil and the pursuit by the orcs, themselves pursued by the elves Legolas and Tauriel are definitely lacking in subtlety!—to spend time on the history of the land, and on the reasons for the behaviour of the elves towards the dwarves, or on the past glory of Dale… The New-Zealand mountain landscapes are as beautiful as ever, but lack in bringing strength to the story, a band of orgs on wargs against a thin ridge in the rising sun replacing a company of dwarves on a moor against a beautiful sunset in the mountains in the previous film. Smaug is also a delicate topic: it is beautifully played by Cumberbatch, who gave more than his voice to the dragon. (And the irony of having Smaug getting the higher ground in his conversation with Bilbo, just like Holmes getting the better of Watson in the BBC series!) Nonetheless, the last third of the film when the dwarves face him is altogether unconvincing, missing the subtle and hypnotic features of dragons and somehow making Smaug appear more like the dragon in Shrek… The trick of the final scene eventually worked out for me, but the preliminaries were so unconvincing. Having Smaug playing hide and seek with the group of dwarves, while destroying the halls of Erebor, is contradicting the reputation for deep cunning (Μῆτις) of the dragons! The last point I want to make is somehow of lesser importance: Peter Jackson chose to move away from the book in many more ways in this second film, when compared with the first one. This is not an issue in that no movie can reproduce the most notable features of the book, so changes would be welcomed had they brought a more epic tone to the Quest. Alas, this is not the case and the scenes of Gandalf in dol Guldur make him sound like an incompetent beginner, while the inclusions of Tauriel and of the corrupted Master of Laketown branch off the main theme with a superfluous love triangle and with an unnecessary depiction of greed, once again taking some precious time off from setting the journey more safely into its epic dimension. Not too mention the additional tension created by the orcish pursuit. All in all, not an unpleasant film, but much lighter than it could have been…

Posterior expectation of regularly paved random histograms

Posted in Books, Statistics, University life with tags , , , , , , , , , on October 7, 2013 by xi'an

Today, Raazesh Sainudiin from the University of Canterbury, in Christchurch, New Zealand, gave a seminar at CREST in our BIP (Bayesians in Paris) seminar series. Here is his abstract:

We present a novel method for averaging a sequence of histogram states visited by a Metropolis-Hastings Markov chain whose stationary distribution is the posterior distribution over a dense space of tree-based histograms. The computational efficiency of our posterior mean histogram estimate relies on a statistical data-structure that is sufficient for non-parametric density estimation of massive, multi-dimensional metric data. This data-structure is formalized as statistical regular paving (SRP). A regular paving (RP) is a binary tree obtained by selectively bisecting boxes along their first widest side. SRP augments RP by mutably caching the recursively computable sufficient statistics of the data. The base Markov chain used to propose moves for the Metropolis-Hastings chain is a random walk that data-adaptively prunes and grows the SRP histogram tree. We use a prior distribution based on Catalan numbers and detect convergence heuristically. The L1-consistency of the the initializing strategy over SRP histograms using a data-driven randomized priority queue based on a generalized statistically equivalent blocks principle is proved by bounding the Vapnik-Chervonenkis shatter coefficients of the class of SRP histogram partitions. The performance of our posterior mean SRP histogram is empirically assessed for large sample sizes simulated from several multivariate distributions that belong to the space of SRP histograms.

The paper actually appeared in the special issue of TOMACS Arnaud Doucet and I edited last year. It is coauthored by Dominic Lee, Jennifer Harlow and Gloria Teng. Unfortunately, Raazesh could not connect to our video-projector. Or fortunately as he gave a blackboard talk that turned to be fairly intuitive and interactive.

summary statistics for ABC model choice

Posted in Statistics with tags , , , , , , , , , on March 11, 2013 by xi'an

countryside near Kenilworth, England, March 5, 2013A few days ago, Dennis Prangle, Paul Fernhead, and their co-authors from New Zealand have posted on arXiv their (long-awaited) study of the selection of summary statistics for ABC model choice. And I read it during my trip to England, in trains and planes, if not when strolling in the beautiful English countryside as above.

As posted several times on this ‘Og, the crux of the analysis is that the Bayes factor is a good type of summary when comparing two models, this result extending to more model by considering instead the vector of evidences. As in the initial Read Paper by Fearnhead and Prangle, there is no true optimality in using the Bayes factor or vector of evidences, strictly speaking, besides the fact that the vector of evidences is minimal sufficient for the marginal models (integrating out the parameters). (This was a point made in my discussion.) The implementation of the principle is similar to this Read Paper setting as well: run a pilot ABC simulation, estimate the vector of evidences, and re-run the main ABC simulation using this estimate as the summary statistic. The paper contains a simulation study using some of our examples (in Marin et al., 2012), as well as an application to genetic bacterial data. Continue reading

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