## Archive for Montréal

## ISBA World meetings to come

Posted in pictures, Statistics, Travel, University life with tags Canada, China, conference, ISBA, Italy, Montréal, Venezia, world meeting on January 27, 2019 by xi'an## a question from McGill about The Bayesian Choice

Posted in Books, pictures, Running, Statistics, Travel, University life with tags Bayes factors, Bayesian hypothesis testing, Canada, cross validated, improper prior, McGill University, Montréal, posterior probability on December 26, 2018 by xi'an**I** received an email from a group of McGill students working on Bayesian statistics and using The Bayesian Choice (although the exercise pictured below is not in the book, the closest being exercise 1.53 inspired from Raiffa and Shlaiffer, 1961, and exercise 5.10 as mentioned in the email):

There was a question that some of us cannot seem to decide what is the correct answer. Here are the issues,

Some people believe that the answer to both is ½, while others believe it is 1. The reasoning for ½ is that since Beta is a continuous distribution, we never could have θ exactly equal to ½. Thus regardless of α, the probability that θ=½ in that case is 0. Hence it is ½. I found a related stack exchange question that seems to indicate this as well.

The other side is that by Markov property and mean of Beta(a,a), as α goes to infinity , we will approach ½ with probability 1. And hence the limit as α goes to infinity for both (a) and (b) is 1. I think this also could make sense in another context, as if you use the Bayes factor representation. This is similar I believe to the questions in the Bayesian Choice, 5.10, and 5.11.

As it happens, the answer is ½ in the first case (a) because π(H⁰) is ½ regardless of α and 1 in the second case (b) because the evidence against H⁰ goes to zero as α goes to zero *(watch out!)*, along with the mass of the prior on any compact of (0,1) since Γ(2α)/Γ(α)². (The limit does not correspond to a proper prior and hence is somewhat meaningless.) However, when α goes to infinity, the evidence against H⁰ goes to infinity and the posterior probability of ½ goes to zero, despite the prior under the alternative being more and more concentrated around ½!

## RNG impact on MCMC [or lack thereof]

Posted in Books, R, Statistics, Travel, University life with tags Donald Knuth, George Marsaglia, GNU C library, MCM 2017, Montréal, R, random number generator, Super-duper on July 13, 2017 by xi'an**F**ollowing the talk at MCM 2017 about the strange impact of the random generator on the outcome of an MCMC generator, I tried in Montréal airport the following code on the banana target of Haario et al. (1999), copied from Soetaert and Laine and using the MCMC function of the FME package:

library(FME) Banana <- function (x1, x2) { return(x2 - (x1^2+1)) } pmultinorm <- function(vec, mean, Cov) { diff <- vec - mean ex <- -0.5*t(diff) %*% solve(Cov) %*% diff rdet <- sqrt(det(Cov)) power <- -length(diff)*0.5 return((2.*pi)^power / rdet * exp(ex)) } BananaSS <- function (p) { P <- c(p[1], Banana(p[1], p[2])) Cov <- matrix(nr = 2, data = c(1, 0.9, 0.9, 1)) N=1e3 ejd=matrix(0,4,N) RNGkind("Mars") for (t in 1:N){ MCMC <- modMCMC(f = BananaSS, p = c(0, 0.7), jump = diag(nrow = 2, x = 5), niter = 1e3) ejd[1,t]=mean((MCMC$pars[-1,2]-MCMC$pars[1,2])^2)}

since this divergence from the initial condition seemed to reflect the experiment of the speaker at MCM 2017. Unsurprisingly, no difference came from using the different RNGs in R (which may fail to contain those incriminated by the study)…

## MCM 2017 snapshots [#2]

Posted in Books, pictures, Running, Statistics, University life with tags ABC, ABC consistency, abcrf, Art Owen, GNU C library, MCM 2017, Mersenne Twisters, Monte Carlo Statistical Methods, Montréal, R, random forests, ratio of uniform algorithm on July 7, 2017 by xi'an**O**n the second day of MCM 2017, Emmanuel Gobet (from Polytechnique) gave the morning plenary talk on regression Monte Carlo methods, where he presented several ways of estimating conditional means of rv’s in nested problems where conditioning involves other conditional expectations. While interested in such problems in connection with ABC, I could not see how the techniques developed therein could apply to said problems.

By some of random chance, I ended up attending a hard-core random generation session where the speakers were discussing discrepancies between GNU library generators [I could not understand the target of interest and using MCMC till convergence seemed prone to false positives!], and failed statistical tests of some 64-bit Mersenne Twisters, and low discrepancy on-line subsamples of Uniform samples. Most exciting of all, Josef Leydold gave a talk on ratio-of-uniforms, on which I spent some time a while ago (till ending up reinventing the wheel!), with highly refined cuts of the original box.

My own 180 slides [for a 50mn talk] somewhat worried my chairman, Art Owen, who kindly enquired the day before at the likelihood I could go through all 184 of them!!! I had appended the ABC convergence slides to an earlier set of slides on ABC with random forests in case of questions about that aspect, although I did not plan to go through those slides [and I mostly covered the 64 other slides] As the talk was in fine more about an inference method than a genuine Monte Carlo technique, plus involved random forests that sounded unfamiliar to many, I did not get many questions from the audience but had several deep discussions with people after the talk. Incidentally, we have just reposted our paper on ABC estimation via random forests, updated the abcrf R package, and submitted it to Peer Community in Evolutionary Biology!

## MCM17 snapshots

Posted in Kids, Mountains, pictures, Running, Statistics, Travel, University life with tags adaptive MCMC methods, local scaling, MCM 2017, Metropolis-within-Gibbs algorithm, Mont Royal, Monte Carlo Statistical Methods, Montréal, Québec, Saint-Laurent, stochastic gradient on July 5, 2017 by xi'an**A**t MCM2017 today, Radu Craiu presented a talk on adaptive Metropolis-within-Gibbs, using a family of proposals for each component of the target and weighting them by jumping distance. And managing the adaptation from the selection rate rather than from the acceptance rate as we did in population Monte Carlo. I find the approach quite interesting in that adaptation and calibration of Metropolis-within-Gibbs is quite challenging due to the conditioning, i.e., the optimality of one scale is dependent on the other components. Some of the graphs produced by Radu during the talk showed a form of local adaptivity that seemed promising. This raised a question I could not ask for lack of time, namely that with a large enough collection of proposals, it is unclear why this approach provides a gain compared with particle, sequential or population Monte Carlo algorithms. Indeed, when there are many parallel proposals, clouds of particles can be generated from all proposals in proportion to their appeal and merged together in an importance manner, leading to an easier adaptation. As it went, the notion of local scaling also reflected in Mylène Bédard’s talk on another Metropolis-within-Gibbs study of optimal rates. The other interesting sessions I attended were the ones on importance sampling with stochastic gradient optimisation, organised by Ingmar Schuster, and on sequential Monte Carlo, with a divide-and-conquer resolution through trees by Lindsten et al. I had missed.