Sobol’s Monte Carlo

The name of Ilya Sobol is familiar to researchers in quasi-Monte Carlo methods for his Sobol’s sequences. I was thus surprised to find in my office a small book entitled The Monte Carlo Method by this author, which is a translation of his 1968 book in Russian. I have no idea how it reached my office and I went to check with the library of Paris-Dauphine around the corner [of my corridor] whether it had been lost: apparently, the library got rid of it among a collection of old books… Now, having read through this 67 pages book (or booklet as Sobol puts it) makes me somewhat agree with the librarians, in that there is nothing of major relevance in this short introduction. It is quite interesting to go through the book and see the basics of simulation principles and Monte Carlo techniques unfolding, from the inverse cdf principle [established by a rather convoluted proof] to importance sampling, but the amount of information is about equivalent to the Wikipedia entry on the topic. From an historical perspective, it is also captivating to see the efforts to connect physical random generators (such as those based on vacuum tube noise) to shift-register pseudo-random generators created by Sobol in 1958. On a Soviet Strela computer.

While Googling the title of that book could not provide any connection, I found out that a 1994 version had been published under the title of A Primer for the Monte Carlo Method, which is mostly the same as my version, except for a few additional sections on pseudo-random generation, from the congruential method (with a FORTRAN code) to the accept-reject method being then called von Neumann’s instead of Neyman’s, to the notion of constructive dimension of a simulation technique, which amounts to demarginalisation, to quasi-Monte Carlo [for three pages]. A funny side note is that the author notes in the preface that the first translation [now in my office] was published without his permission!

winning entry at MCqMC’16

The nice logo of MCqMC 2016 was a collection of eight series of QMC dots on the unit (?) cube. The organisers set a competition to identify the principles behind those quasi-random sets and as I had no idea for most of them I entered very random sets unconnected with algorithmia, for which I got an honourable mention and a CD prize (if not the conference staff tee-shirt I was coveting!) Art Owen sent me back my entry, posted below and hopefully (or not!) readable.

MCMskv #4 [house with a vision]

Last day at MCMskv! Not yet exhausted by this exciting conference, but this was the toughest day with one more session and a tutorial by Art Own on quasi Monte-Carlo. (Not even mentioning the night activities that I skipped. Or the ski break that I did not even consider.) Krys Latunszynski started with a plenary on exact methods for discretised diffusions, with a foray in Bernoulli factory problems. Then a neat session on adaptive MCMC methods that contained a talk by Chris Sherlock on delayed acceptance, where the approximation to the target was built by knn trees. (The adaptation was through the construction of the tree by including additional evaluations of the target density. Another paper sitting in my to-read list for too a long while: the exploitation of the observed values of π towards improving an MCMC sampler has always be “obvious” to me even though I could not see any practical way of doing so. )

It was wonderful that Art Owen accepted to deliver a tutorial at MCMskv on quasi-random Monte Carlo. Great tutorial, with a neat coverage of the issues most related to Monte Carlo integration. Since quasi-random sequences have trouble with accept/reject methods, a not-even-half-baked idea that came to me during Art’s tutorial was that the increased computing power granted by qMC could lead to a generic integration of the Metropolis-Hastings step in a Rao-Blackwellised manner. Art mentioned he was hoping that in a near future one could switch between pseudo- and quasi-random in an almost automated manner when running standard platforms like R. This would indeed be great, especially since quasi-random sequences seem to be available at the same cost as their pseudo-random counterpart. During the following qMC session, Art discussed the construction of optimal sequences on sets other than hypercubes (with the surprising feature that projecting optimal sequences from the hypercube does not work). Mathieu Gerber presented the quasi-random simulated annealing algorithm he developed with Luke Bornn that I briefly discussed a while ago. Or thought I did as I cannot trace a post on that paper! While the fact that annealing also works with quasi-random sequences is not astounding, the gain over random sequences shown on two examples is clear. The session also had a talk by Lester Mckey who relies Stein’s discrepancy to measure the value of an approximation to the true target. This was quite novel, with a surprising connection to Chris Oates’ talk and the use of score-based control variates, if used in a dual approach.

Another great session was the noisy MCMC one organised by Paul Jenkins (Warwick), with again a coherent presentation of views on the quality or lack thereof of noisy (or inexact) versions, with an update from Richard Everitt on inexact MCMC, Felipe Medina Aguayo (Warwick) on sufficient conditions for noisy versions to converge (and counterexamples), Jere Koskela (Warwick) on a pseudo-likelihood approach to the highly complex Kingman’s coalescent model in population genetics (of ABC fame!), and Rémi Bardenet on the tall data approximations techniques discussed in a recent post. Having seen or read most of those results previously did not diminish the appeal of the session.

Advances in scalable Bayesian computation [day #4]

Final day of our workshop Advances in Scalable Bayesian Computation already, since tomorrow morning is an open research time ½ day! Another “perfect day in paradise”, with the Banff Centre campus covered by a fine snow blanket, still falling…, and making work in an office of BIRS a dream-like moment.

Still looking for a daily theme, parallelisation could be the right candidate, even though other talks this week went into parallelisation issues, incl. Steve’s talk yesterday. Indeed, Anthony Lee gave a talk this morning on interactive sequential Monte Carlo, where he motivated the setting by a formal parallel structure. Then, Darren Wilkinson surveyed the parallelisation issues in Monte Carlo, MCMC, SMC and ABC settings, before arguing in favour of a functional language called Scala. (Neat entries to those topics can be found on Darren’s blog.) And in the afternoon session, Sylvia Frühwirth-Schnatter exposed her approach to the (embarrassingly) parallel problem, in the spirit of Steve’s , David Dunson’s and Scott’s (a paper posted on the day I arrived in Chamonix and hence I missed!). There was plenty to learn from that talk (do not miss the Yin-Yang moment at 25 mn!), but it also helped me to break a difficulty I had with the consensus Bayes representation for two weeks (more on that later!). And, even though Marc Suchard mostly talked about flu and trees in a very pleasant and broad talk, he also had a slide on parallelisation to fit the theme! Although unrelated with parallelism,  Nicolas Chopin’s talk was on sequential quasi-Monte Carlo algorithms: while I had heard previous versions of this talk in Chamonix and BigMC, I found it full of exciting stuff. And it clearly got the room truly puzzled by this possibility, in a positive way! Similarly, Alex Lenkoski spoke about extreme rain events in Norway with no trace of parallelism, but the general idea behind the examples was to question the notion of the calibrated Bayesian (with possible connections with the cut models).

This has been a wonderful week and I am sure the participants got as much as I did from the talks and the informal exchanges. Thanks to BIRS for the sponsorship and the superb organisation of the week (and to the Banff Centre for providing such a paradisical environment). I feel very privileged to have benefited from this support, even though I deadly hope to be back in Banff within a few years.

A survey of the [60′s] Monte Carlo methods [2]

The 24 questions asked by John Halton in the conclusion of his 1970 survey are

1. Can we obtain a theory of convergence for random variables taking values in Fréchet spaces?
2. Can the study of Monte Carlo estimates in separable Fréchet spaces give a theory of global approximation?
3. When sampling functions, what constitutes a representative sample of function values?
4. Can one apply Monte Carlo to pattern recognition?
5. Relate Monte Carlo theory to the theory of random equations.
6. What can be said about quasi-Monte Carlo estimates for finite-dimensional and infinite-dimensional integrals?
7. Obtain expression, asymptotic forms or upper bounds for L² and L^∞ discrepancies of quasirandom sequences.
8. How should one improve quasirandom sequences?
9. How to interpret the results of statistical tests applied to pseudo- or quasirandom sequences?
10. Can we develop a meaningful statistical theory of quasi-Monte Carlo estimates?
11. Can existing Monte Carlo techniques be improved and applied to new classes of problems?
12. Can the design of Monte Carlo estimators be made more systematic?
13. How can the idea of sequential Monte Carlo be extended?
14. Can sampling with signed probabilities be made practical?
15. What is the best allocation effort in obtaining zeroth- and first-level estimators in algebraic problems?
16. Examine the Monte Carlo analogues of the various matrix iterative schemes.
17. Develop the schemes of grid refinement in continuous problems.
18. Develop new Monte Carlo eigenvectors and eigenvalue techniques.
19. Develop fast, reliable true canonical random generators.
20. How is the output of a true random generator to be tested?
21. Develop fast, efficient methods for generating arbitrary random generators.
22. Can we really have useful general purpose pseudorandom sequences.
23. What is the effect of the discreteness of digital computers on Monte Carlo calculations?
24. Is there a way to estimate the accuracy of Monte Carlo estimates?