Archive for CHANCE

Bangalore workshop [ಬೆಂಗಳೂರು ಕಾರ್ಯಾಗಾರ] and new book

Posted in Books, pictures, R, Statistics, Travel, University life with tags , , , , , , , , , , , , on August 13, 2014 by xi'an

IIScOn the last day of the IFCAM workshop in Bangalore, Marc Lavielle from INRIA presented a talk on mixed effects where he illustrated his original computer language Monolix. And mentioned that his CRC Press book on Mixed Effects Models for the Population Approach was out! (Appropriately listed as out on a 14th of July on amazon!) He actually demonstrated the abilities of Monolix live and on diabets data provided by an earlier speaker from Kolkata, which was a perfect way to start initiating a collaboration! Nice cover (which is all I saw from the book at this stage!) that maybe will induce candidates to write a review for CHANCE. Estimation of those mixed effect models relies on stochastic EM algorithms developed by Marc Lavielle and Éric Moulines in the 90’s, as well as MCMC methods.

JSM 2014, Boston [#2]

Posted in Statistics, Travel, University life with tags , , , , , , , , on August 7, 2014 by xi'an

Day #2 at JSM started quite early as I had to be on site by 7am for the CHANCE editors breakfast. No running then, except to Porter metro station. Interesting exchange full of new ideas to keep the journal cruising. In particular, a call for proposals on special issues on sexy topics (reproducible research anyone? I already have some book reviews.). And directions to increase the international scope and readership. And possibly adding or reporting on a data challenge. After this great start, I attended the Bayesian Time Series and Dynamic Models session, where David Scott Matteson from Cornell University presented an extension of the Toronto ambulance data analysis Dawn Woodard had exposed in Banff at an earlier workshop. The extension dealt with the spatio-temporal nature of the data,  using a mixture model with time-dependent weights that revolved cyclically in an autoexponential manner. And rekindling the interest in the birth-and-death alternative to reversible jump. Plus another talk by Scott Holan mixing Bayesian analysis with frequency data, an issue that always puzzled me. The second session I attended was Multiscale Modeling for Complex Massive Data, with a modelling of brain connections through a non-parametric mixture by David Dunson. And a machine learning talk by Mauro Maggioni on a projection cum optimisation technique to fight the curse of dimension. Who proposed a solution to an optimal transport problem that is much more convincing than the one I discussed a while ago. Unfortunately, this made me miss the Biometrics showcase session, where Debashis Mondal presented a joint work with Julian Besag on Exact Goodness-of-Fit Tests for Markov Chains. And where both my friends Michael Newton and Peter Green were discussants… An idle question that came to me during this last talk was about the existence of particle filters for spatial Markov structures (rather than the usual ones on temporal Markov models).

After a [no] lunch break spent on pondering over a conjecture laid to me by Natesh Pillai yesterday, I eventually joined the Feature Allocation session. Eventually as I basically had to run the entire perimeter of the conference centre! The three talks by Finale Doshi-Velez, Tamara Broderick, and Yuan Ji were all impressive and this may have been my best session so far at JSM! Thanks to Peter Müller for organising it! Tamara Broderick focussed on a generic way to build conjugate priors for non-parametric models, with all talks involving Indian buffets. Maybe a suggestion for tonight’s meal..! (In the end, great local food onn Harvard Square.)

implementing reproducible research [short book review]

Posted in Books, Kids, pictures, R, Statistics, Travel, University life with tags , , , , , , , , , , , on July 15, 2014 by xi'an

As promised, I got back to this book, Implementing reproducible research (after the pigeons had their say). I looked at it this morning while monitoring my students taking their last-chance R exam (definitely last chance as my undergraduate R course is not reconoduced next year). The book is in fact an edited collection of papers on tools, principles, and platforms around the theme of reproducible research. It obviously links with other themes like open access, open data, and open software. All positive directions that need more active support from the scientific community. In particular the solutions advocated through this volume are mostly Linux-based. Among the tools described in the first chapter, knitr appears as an alternative to sweave. I used the later a while ago and while I like its philosophy. it does not extend to situations where the R code within takes too long to run… (Or maybe I did not invest enough time to grasp the entire spectrum of sweave.) Note that, even though the book is part of the R Series of CRC Press, many chapters are unrelated to R. And even more [unrelated] to statistics.

This limitation is somewhat my difficulty with [adhering to] the global message proposed by the book. It is great to construct such tools that monitor and archive successive versions of code and research, as anyone can trace back the research steps conducting to the published result(s). Using some of the platforms covered by the book establishes for instance a superb documentation principle, going much further than just providing an “easy” verification tool against fraudulent experiments. The notion of a super-wiki where notes and preliminary versions and calculations (and dead ends and failures) would be preserved for open access is just as great. However this type of research processing and discipline takes time and space and human investment, i.e. resources that are sparse and costly. Complex studies may involve enormous amounts of data and, neglecting the notions of confidentiality and privacy, the cost of storing such amounts is significant. Similarly for experiments that require days and weeks of huge clusters. I thus wonder where those resources would be found (journals, universities, high tech companies, …?) for the principle to hold in full generality and how transient they could prove. One cannot expect the research time to garantee availability of those meta-documents for remote time horizons. Just as a biased illustration, checking the available Bayes’ notebooks meant going to a remote part of London at a specific time and with a preliminary appointment. Those notebooks are not available on line for free. But for how long?

“So far, Bob has been using Charlie’s old computer, using Ubuntu 10.04. The next day, he is excited to find the new computer Alice has ordered for him has arrived. He installs Ubuntu 12.04″ A. Davison et al.

Putting their principles into practice, the authors of Implementing reproducible research have made all chapters available for free on the Open Science Framework. I thus encourage anyone interesting in those principles (and who would not be?!) to peruse the chapters and see how they can benefit from and contribute to open and reproducible research.

revenge of the pigeons

Posted in Books, Kids, pictures, R, Statistics, Travel, University life with tags , , , , , , on June 24, 2014 by xi'an

pigeonpooWhile I had not had kamikaze pigeons hitting my windows for quite a while…, it may be that one of them decided to move to biological warfare: when I came back from Edinburgh, my office at the University was in a terrible state as a bird had entered through a tiny window opening and wrecked havoc on the room, dropping folders and rocks from my shelves and… leaving a most specific proof of its visit. This bird was particularly attracted by and aggressive against the above book, Implementing Reproducible Research, standing on top of my books to review for CHANCE. Obvious disclaimer: this reflects neither my opinion nor the University opinion about the book contents, but only the bird’s, which is solely responsible for its action!

Bayesian programming [book review]

Posted in Books, Kids, pictures, Statistics, University life with tags , , , , , , , , , , on March 3, 2014 by xi'an

“We now think the Bayesian Programming methodology and tools are reaching maturity. The goal of this book is to present them so that anyone is able to use them. We will, of course, continue to improve tools and develop new models. However, pursuing the idea that probability is an alternative to Boolean logic, we now have a new important research objective, which is to design specific hsrdware, inspired from biology, to build a Bayesian computer.”(p.xviii)

On the plane to and from Montpellier, I took an extended look at Bayesian Programming a CRC Press book recently written by Pierre Bessière, Emmanuel Mazer, Juan-Manuel Ahuactzin, and Kamel Mekhnacha. (Very nice picture of a fishing net on the cover, by the way!) Despite the initial excitement at seeing a book which final goal was to achieve a Bayesian computer, as demonstrated by the above quote, I however soon found the book too arid to read due to its highly formalised presentation… The contents are clear indications that the approach is useful as they illustrate the use of Bayesian programming in different decision-making settings, including a collection of Python codes, so it brings an answer to the what but it somehow misses the how in that the construction of the priors and the derivation of the posteriors is not explained in a way one could replicate.

“A modeling methodology is not sufficient to run Bayesian programs. We also require an efficient Bayesian inference engine to automate the probabilistic calculus. This assumes we have a collection of inference algorithms adapted and tuned to more or less specific models and a software architecture to combine them in a coherent and unique tool.” (p.9)

For instance, all models therein are described via the curly brace formalism summarised by

phdthesis28xwhich quickly turns into an unpalatable object, as in this example taken from the online PhD thesis of Gabriel Synnaeve (where he applied Bayesian programming principles to a MMORPG called StarCraft and developed an AI (or bot) able to play BroodwarBotQ)

phdthesis37xthesis that I found most interesting!

“Consequently, we have 21 × 16 = 336 bell-shaped distributions and we have 2 × 21 × 16 = 772 free parameters: 336 means and 336 standard deviations.¨(p.51)

Now, getting back to the topic of the book, I can see connections with statistical problems and models, and not only via the application of Bayes’ theorem, when the purpose (or Question) is to take a decision, for instance in a robotic action. I still remain puzzled by the purpose of the book, since it starts with very low expectations on the reader, but hurries past notions like Kalman filters and Metropolis-Hastings algorithms in a few paragraphs. I do not get some of the details, like this notion of a discretised Gaussian distribution (I eventually found the place where the 772 prior parameters are “learned” in a phase called “identification”.)

“Thanks to conditional independence the curse of dimensionality has been broken! What has been shown to be true here for the required memory space is also true for the complexity of inferences. Conditional independence is the principal tool to keep the calculation tractable. Tractability of Bayesian inference computation is of course a major concern as it has been proved NP-hard (Cooper, 1990).”(p.74)

The final chapters (Chap. 14 on “Bayesian inference algorithms revisited”, Chap. 15 on “Bayesian learning revisited” and  Chap. 16 on “Frequently asked questions and frequently argued matters” [!]) are definitely those I found easiest to read and relate to. With mentions made of conjugate priors and of the EM algorithm as a (Bayes) classifier. The final chapter mentions BUGS, Hugin and… Stan! Plus a sequence of 23 PhD theses defended on Bayesian programming for robotics in the past 20 years. And explains the authors’ views on the difference between Bayesian programming and Bayesian networks (“any Bayesian network can be represented in the Bayesian programming formalism, but the opposite is not true”, p.316), between Bayesian programming and probabilistic programming (“we do not search to extend classical languages but rather to replace them by a new programming approach based on probability”, p.319), between Bayesian programming and Bayesian modelling (“Bayesian programming goes one step further”, p.317), with a further (self-)justification of why the book sticks to discrete variables, and further more philosophical sections referring to Jaynes and the principle of maximum entropy.

“The “objectivity” of the subjectivist approach then lies in the fact that two different subjects with same preliminary knowledge and same observations will inevitably reach the same conclusions.”(p.327)

Bayesian Programming thus provides a good snapshot of (or window on) what one can achieve in uncertain environment decision-making with Bayesian techniques. It shows a long-term reflection on those notions by Pierre Bessière, his colleagues and students. The topic is most likely too remote from my own interests for the above review to be complete. Therefore, if anyone is interested in reviewing any further this book for CHANCE, before I send the above to the journal, please contact me. (Usual provisions apply.)

Nonlinear Time Series just appeared

Posted in Books, R, Statistics, University life with tags , , , , , , , , , , , , , , , on February 26, 2014 by xi'an

My friends Randal Douc and Éric Moulines just published this new time series book with David Stoffer. (David also wrote Time Series Analysis and its Applications with Robert Shumway a year ago.) The books reflects well on the research of Randal and Éric over the past decade, namely convergence results on Markov chains for validating both inference in nonlinear time series and algorithms applied to those objects. The later includes MCMC, pMCMC, sequential Monte Carlo, particle filters, and the EM algorithm. While I am too close to the authors to write a balanced review for CHANCE (the book is under review by another researcher, before you ask!), I think this is an important book that reflects the state of the art in the rigorous study of those models. Obviously, the mathematical rigour advocated by the authors makes Nonlinear Time Series a rather advanced book (despite the authors’ reassuring statement that “nothing excessively deep is used”) more adequate for PhD students and researchers than starting graduates (and definitely not advised for self-study), but the availability of the R code (on the highly personal page of David Stoffer) comes to balance the mathematical bent of the book in the first and third parts. A great reference book!

Bayes’ Theorem in the 21st Century, really?!

Posted in Books, Statistics with tags , , , , , , on June 20, 2013 by xi'an

“In place of past experience, frequentism considers future behavior: an optimal estimator is one that performs best in hypothetical repetitions of the current experiment. The resulting gain in scientific objectivity has carried the day…”

Julien Cornebise sent me this Science column by Brad Efron about Bayes’ theorem. I am a tad surprised that it got published in the journal, given that it does not really contain any new item of information. However, being unfamiliar with Science, it may also be that it also publishes major scientists’ opinions or warnings, a label that can fit this column in Science. (It is quite a proper coincidence that the post appears during Bayes 250.)

Efron’s piece centres upon the use of objective Bayes approaches in Bayesian statistics, for which Laplace was “the prime violator”. He argues through examples that noninformative “Bayesian calculations cannot be uncritically accepted, and should be checked by other methods, which usually means “frequentistically”. First, having to write “frequentistically” once is already more than I can stand! Second, using the Bayesian framework to build frequentist procedures is like buying top technical outdoor gear to climb the stairs at the Sacré-Coeur on Butte Montmartre! The naïve reader is then left clueless as to why one should use a Bayesian approach in the first place. And perfectly confused about the meaning of objectivity. Esp. given the above quote! I find it rather surprising that this old saw of a  claim of frequentism to objectivity resurfaces there. There is an infinite range of frequentist procedures and, while some are more optimal than others, none is “the” optimal one (except for the most baked-out examples like say the estimation of the mean of a normal observation).

“A Bayesian FDA (there isn’t one) would be more forgiving. The Bayesian posterior probability of drug A’s superiority depends only on its final evaluation, not whether there might have been earlier decisions.”

The second criticism of Bayesianism therein is the counter-intuitive irrelevance of stopping rules. Once again, the presentation is fairly biased, because a Bayesian approach opposes scenarii rather than evaluates the likelihood of a tail event under the null and only the null. And also because, as shown by Jim Berger and co-authors, the Bayesian approach is generally much more favorable to the null than the p-value.

“Bayes’ Theorem is an algorithm for combining prior experience with current evidence. Followers of Nate Silver’s FiveThirtyEight column got to see it in spectacular form during the presidential campaign: the algorithm updated prior poll results with new data on a daily basis, nailing the actual vote in all 50 states.”

It is only fair that Nate Silver’s book and column are mentioned in Efron’s column. Because it is a highly valuable and definitely convincing illustration of Bayesian principles. What I object to is the criticism “that most cutting-edge science doesn’t enjoy FiveThirtyEight-level background information”. In my understanding, the poll model of FiveThirtyEight built up in a sequential manner a weight system over the different polling companies, hence learning from the data if in a Bayesian manner about their reliability (rather than forgetting the past). This is actually what caused Larry Wasserman to consider that Silver’s approach was actually more frequentist than Bayesian…

“Empirical Bayes is an exciting new statistical idea, well-suited to modern scientific technology, saying that experiments involving large numbers of parallel situations carry within them their own prior distribution.”

My last point of contention is about the (unsurprising) defence of the empirical Bayes approach in the Science column. Once again, the presentation is biased towards frequentism: in the FDR gene example, the empirical Bayes procedure is motivated by being the frequentist solution. The logical contradiction in “estimat[ing] the relevant prior from the data itself” is not discussed and the conclusion that Brad Efron uses “empirical Bayes methods in the parallel case [in the absence of prior information”, seemingly without being cautious and “uncritically”, does not strike me as the proper last argument in the matter! Nor does it give a 21st Century vision of what nouveau Bayesianism should be, faced with the challenges of Big Data and the like…

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