## Bayes’ Rule [book review]

Posted in Books, Statistics, University life with tags , , , , , , , , , , on July 10, 2014 by xi'an

This introduction to Bayesian Analysis, Bayes’ Rule, was written by James Stone from the University of Sheffield, who contacted CHANCE suggesting a review of his book. I thus bought it from amazon to check the contents. And write a review.

First, the format of the book. It is a short paper of 127 pages, plus 40 pages of glossary, appendices, references and index. I eventually found the name of the publisher, Sebtel Press, but for a while thought the book was self-produced. While the LaTeX output is fine and the (Matlab) graphs readable, pictures are not of the best quality and the display editing is minimal in that there are several huge white spaces between pages. Nothing major there, obviously, it simply makes the book look like course notes, but this is in no way detrimental to its potential appeal. (I will not comment on the numerous appearances of Bayes’ alleged portrait in the book.)

“… (on average) the adjusted value θMAP is more accurate than θMLE.” (p.82)

Bayes’ Rule has the interesting feature that, in the very first chapter, after spending a rather long time on Bayes’ formula, it introduces Bayes factors (p.15).  With the somewhat confusing choice of calling the prior probabilities of hypotheses marginal probabilities. Even though they are indeed marginal given the joint, marginal is usually reserved for the sample, as in marginal likelihood. Before returning to more (binary) applications of Bayes’ formula for the rest of the chapter. The second chapter is about probability theory, which means here introducing the three axioms of probability and discussing geometric interpretations of those axioms and Bayes’ rule. Chapter 3 moves to the case of discrete random variables with more than two values, i.e. contingency tables, on which the range of probability distributions is (re-)defined and produces a new entry to Bayes’ rule. And to the MAP. Given this pattern, it is not surprising that Chapter 4 does the same for continuous parameters. The parameter of a coin flip.  This allows for discussion of uniform and reference priors. Including maximum entropy priors à la Jaynes. And bootstrap samples presented as approximating the posterior distribution under the “fairest prior”. And even two pages on standard loss functions. This chapter is followed by a short chapter dedicated to estimating a normal mean, then another short one on exploring the notion of a continuous joint (Gaussian) density.

“To some people the word Bayesian is like a red rag to a bull.” (p.119)

Bayes’ Rule concludes with a chapter entitled Bayesian wars. A rather surprising choice, given the intended audience. Which is rather bound to confuse this audience… The first part is about probabilistic ways of representing information, leading to subjective probability. The discussion goes on for a few pages to justify the use of priors but I find completely unfair the argument that because Bayes’ rule is a mathematical theorem, it “has been proven to be true”. It is indeed a maths theorem, however that does not imply that any inference based on this theorem is correct!  (A surprising parallel is Kadane’s Principles of Uncertainty with its anti-objective final chapter.)

All in all, I remain puzzled after reading Bayes’ Rule. Puzzled by the intended audience, as contrary to other books I recently reviewed, the author does not shy away from mathematical notations and concepts, even though he proceeds quite gently through the basics of probability. Therefore, potential readers need some modicum of mathematical background that some students may miss (although it actually corresponds to what my kids would have learned in high school). It could thus constitute a soft entry to Bayesian concepts, before taking a formal course on Bayesian analysis. Hence doing no harm to the perception of the field.

## [h]it figures

Posted in Books, pictures with tags , , , , , on June 1, 2014 by xi'an

Just a few figures from wordpress about the ‘Og:

• 2,845 posts;
• 1,009,428 views;
• 5,095 tags;
• 1,001 spams in the past 24 hours;
• and… only 5 amazon orders in the past month!

## AISTATS 2014 (day #1)

Posted in Mountains, pictures, Running, Statistics, Travel, University life with tags , , , , , on April 23, 2014 by xi'an

First day at AISTATS 2014! After three Icelandic vacations days driving (a lot) and hinkg (too little) around South- and West-Iceland, I joined close to 300 attendees for this edition of the AISTATS conference series. I was quite happy to be there, if only because I had missed the conference last year (in Phoenix) and did not want this to become a tradition… Second, the mix of statistics, artificial intelligence and machine learning that characterises this conference is quite exciting, if challenging at time. What I most appreciated in this discovery of the conference is the central importance of the poster session, most talks being actually introductions to or oral presentations of posters! I find this feature terrific enough (is there such a notion as “terrific enough”?!) worth adopting in future conferences I am involved in. I just wish I had managed to tour the whole collection of posters today… The (first and) plenary lecture was delivered by Peter Bühlman, who spoke about a compelling if unusual (for me) version of causal inference. This was followed by sessions on Gaussian processes, graphical models, and mixed data sources. One highlight talk was the one by Marc Deisenroth, who showed impressive robotic fast learning based on Gaussian processes. At the end of this full day, I also attended an Amazon mixer where I learned about Amazon‘s entry on the local market, where it seems the company is getting a better picture of the current and future state of the U.S. economy than governmental services, thanks to a very fine analysis of the sales and entries on Amazon‘s entry. Then it was time to bike “home” on my rental bike, in the setting sun…

## Den of thieves [book review]

Posted in Books, Kids, pictures with tags , , , on April 20, 2014 by xi'an

Last month, I ordered several books on amazon,  taking advantage of my amazon associate gains, and some of them were suggested by amazon algorithms based on my recent history. As I had recently read books involving thieves (like Giant Thief, or Broken Blade and the subsequent books), a lot of titles involved thieves or thievery related names… I picked Den of Thieves mainly for its cover as I did not know the author and the story sounded rather common. When I started reading the book, the story got more and more common, pertaining more to an extended Dungeons & Dragons scenario than to a genuine book! The theme of a bright young thief emerging from the gritty underworld of a close city has been over and over exploited in the fantasy literature, the best (?) example being The lies of Locke Lamora. (Whose third volume, The Republic of Thieves, is in my bag for Reykjavik!) This time, the thief does not appear particularly bright, except at times when he starts philosophy-sing with extremely dangerous enemies!, and the way he eventually overcomes insanely unbalanced odds is just too much. Most characters in the novel are not particularly engaging and way too much caricaturesque from the terribly evil sorcerer cavorting with she-demons to the rigid knight sticking to an idealistic vision of the world where ‘honour” and the code of chivalry is the solution to all problems. It is not even in the slightest sarcastic or tongue-in-cheek as the many novels by David Eddings and the main characters are mostly humourless. I wonder why the book did not get better edited as the weaknesses are very easy to spot! A good example where amazon software failed to make a worthy recommendation!

## Clockers [book review]

Posted in Books, Travel with tags , , , , , , , , , on March 15, 2014 by xi'an

Throughout my recent trip to Canada, I read bits and pieces of Clockers by Richard Price and I finished reading it last Sunday. It is an impressive piece of literature and I am surprised I was not aware of its existence until amazon.com suggested it to me (as I was checking for recent books by another Richard, Richard Morgan!). Guessing from the summary it could be of interest and from comments it was sort of a classic, I ordered it more or less on a whim (given a comfortable balance on my amazon.com account, thanks to ‘Og’s readers!) It took me a few pages to realise the plot was deeply set in the 1990’s, not only because this was the high of the crack epidemics, but also since the characters (drug dealers and policemen) therein are all using beepers, instead of cellphones, and street phone booths).

“It’s like a math problem. Juan got whacked at point X, he drove away losing blood at the rate of a pint every ninety seconds. He was driving forty-five miles an hour and he bought the farm two miles inside the tunnel (…) So for ten points, [who] in what New Jersey town did Juan?” Clockers (p.272)

The plot of Clockers is vaguely a detective story as an aging and depressed homicide officer, Rosso, hunts the murderer of a drug dealer, being convinced from the start that the self-declared murderer Victor did not do it. In parallel, and somewhat more closely, the book follows the miserable plight and thoughts and desires of Victor’s brother, Strike, who is head of a local crack dealing network, under the domination of the charismatic and berserk Rodney Little… But the resolution of the crime matters very little, much less than the exposure of the deadly economics of the drug traffic in inner cities (years before Freakonomics!), of the constant fight of single mothers to bring food and structure to their dysfunctional families, to the widespread recourse to moonlighting, and above all to the almost physical impossibility to escape one’s environment (even for smart and decent kids like Victor and, paradoxically enough, the drug-dealing Strike) by lack of prospect and exposure to anything or anywhere else, as well as social pressure, early pregnancies and gang-related micro-partitioning of cities.

When I mentioned Clockers to Andrew, he told me that he also liked it very much but that the characters were not quite “real”. I somewhat agree in that, while the economics, the sociology and the practice of drug-dealing sound very accurately reproduced (for all I know!), the characters are more caricaturesque or picturesque than natural. The stomach disease of Strike sounds too much like an allegory of both his schizophrenic split between running the drug trade and looking for a definitive quit, while the sacrifice of his brother makes little sense, except as a form either of suicide or of escape from an environment he can no longer stand. What is most surprising is that Richard Price (just like Michael Crichton) is  a practised screenwriter (who collaborated to Spike Lee’s 1995 Clockers). So he knows how to run an efficient story with convincing characters and plot(s). Hence my little theory of a picaresque novel… (Here is Jim Shepard’s enthusiastic review of Clockers. With the definitely accurate title of “Sympathy for the dealer”.)

## amazonly associated thanks (& warnin’)

Posted in Books, Kids, R, Statistics with tags , , , , , , , on December 8, 2013 by xi'an

Following a now well-established pattern, let me (re)warn (the few) unwary ‘Og readers that the links to Amazon.com and to Amazon.fr found on this blog are actually susceptible to earn me a monetary gain [from 4% to 8% on the sales] if a purchase is made by the reader in the 24 hours following the entry on Amazon through this link, thanks to the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com/fr. Unlike the pattern of last year, and of the year before last, the mostly purchased item through the links happens to be related to a blog post, since it is Andrew’s book, with 318 copies of its third edition sold through the ‘Og last month! Here are some of the most exotic purchases:

As usual the books I actually reviewed along the past months, positively or negatively, were among the top purchases… Like two dozen copies of The BUGS book. And a dozen of R for dummies. And even a few of The Cartoon Introduction to Statistics. (Despite a most critical review.) Thanks to all of you using those links (for feeding further my book addiction, books that now eventually end up in the math common room in Dauphine or Warwick, once I have read them)!

## machine learning as buzzword

Posted in Books, Kids with tags , , , , , on November 12, 2013 by xi'an

In one of his posts, my friend Larry mentioned that popular posts had to mention the Bayes/frequentist opposition in the title… I think mentioning machine learning is also a good buzzword to increase the traffic! I did spot this phenomenon last week when publishing my review of Kevin Murphy’s Machine Learning: the number of views and visitors jumped by at least a half, exceeding the (admittedly modest) 1000 bar on two consecutive days. Interestingly, the number of copies of Machine Learning (sold via my amazon associate link) did not follow this trend: so far, I only spotted a few copies sold, in similar amounts to the number of copies of Spatio-temporal Statistics I reviewed the week before. Or most books I review, positively or negatively! (However, I did spot a correlated increase in overall amazon associate orderings and brazenly attributed the command of a Lego robotic set to a “machine learner”! And as of yesterday Og‘s readers massively ordered 152 236 copies of the latest edition of Andrew’s Bayesian Data Analysis, Thanks!)