University life

Bayes on drugs (guest post)

May 20, 2012 | xi'an

This post is written by Julien Cornebise. Last week in Aachen was the 3rd Edition of the Bayes(Pharma) workshop. Its specificity: half-and-half industry/academic participants and speakers, all in Pharmaceutical statistics, with a great care to welcome newcomers to Bayes, so as to spread as much as possible the ... [Read more...]

garch() uncertainty

May 16, 2012 | xi'an

As part of an on-going paper with Kerrie Mengersen and Pierre Pudlo, we are using a GARCH(1,1) model as a target. Thus, the model is of the form which is a somehow puzzling object: the latent (variance) part is deterministic and can be reconstructed exactly given the series and the ... [Read more...]

generalised ratio of uniforms

May 14, 2012 | xi'an

A recent arXiv posting of the paper “On the Generalized Ratio of Uniforms as a Combination of Transformed Rejection and Extended Inverse of Density Sampling” by Martino, Luengo, and Míguez from Madrid rekindled my interest in this rather peculiar simulation method. The ratio of uniforms samples uniformly on the ... [Read more...]

ASA fellows

May 12, 2012 | xi'an

Being freshly elected ASA Fellow (yay!), I just received the list of 2012 ASA Fellows. Among whose, let me mention Sudipto Banerjee, University of Minnesota, Minneapolis, Minnesota, elected “For theoretical, methodological and applied research in spatiotemporal statistical modeling, especially as applied to problems in environmetrics, ecology, occupational health, agriculture and economics, ... [Read more...]

relevant, revised, & resubmitted

May 7, 2012 | xi'an

We have now completed our revision of the paper Relevant statistics for Bayesian model choice, written with Judith Rousseau, Jean-Michel Marin, and Natesh Pillai. It has been resubmitted to Series B and reposted on arXiv. The major change in the paper is the inclusion of a check about the relevance ... [Read more...]

what’s wrong with package comment?!

May 3, 2012 | xi'an

I spent most of the Sunday afternoon trying to understand why defining did not have the same effect as writing the line until I found there is a clash due to the comment package… The assuredly simple code produces an error message: This is quite an inconvenience as I need ... [Read more...]

mad statistic

April 29, 2012 | xi'an

In the motivating toy example to our ABC model choice paper, we compare summary statistics, mean, median, variance, and… median absolute deviation (mad). The latest is the only one able to discriminate between our normal and Laplace models (as now discussed on Cross Validated!). When rerunning simulations to produce nicer ... [Read more...]

[not] Le Monde puzzle (solution)

April 13, 2012 | xi'an

Following the question on dinner table permutations on StackExchange (mathematics) and the reply that the right number was six, provided by hardmath, I was looking for a constructive solution how to build the resolvable 2-(20,5,1) covering. A few hours later. hardmath again came up with an answer, found in the ... [Read more...]

simulated annealing for Sudokus [2]

March 16, 2012 | xi'an

On Tuesday, Eric Chi and Kenneth Lange arXived a paper on a comparison of numerical techniques for solving sudokus. (The very Kenneth Lange who wrote this fantastic book on numerical analysis.) One of these techniques is the simulated annealing approach I had played with a long while ago.  They seem ... [Read more...]

IS vs. self-normalised IS

March 11, 2012 | xi'an

I was grading my Master projects this morning and came upon this graph: which compares the variability of an importance-sampling estimator versus its self-normalised alternative… This is an interesting case in that self-normalisation does considerably degrade the quality of the approximation in that setting. In other cases, self-normalisation may bring ... [Read more...]

di Roma

March 4, 2012 | xi'an

It has been a wonderful week in Roma, a mix of pleasant work and enjoyable free-time! I gave the ABC advanced course for the second time in a month so it did not require much in terms of preparation and there was a good sized audience with attentive (if too ... [Read more...]

ABC in Roma [R lab #2]

March 2, 2012 | xi'an

Here are the R codes of the second R lab organised by Serena Arima in supplement of my lectures (now completed!). This morning I covered ABC model choice and the following example is the benchmark used in the course (and in the paper) about the impact of summary statistics. (Warning! ... [Read more...]

ABC in Roma [R lab #1]

February 29, 2012 | xi'an

Here are the R codes of the R labs organised by Serena Arima in supplement of my lectures. This is quite impressive and helpful to the students, as illustrated by the first example below (using the abc software). I am having a great time teaching this “ABC in Roma” course, ... [Read more...]

A Roma

February 25, 2012 | xi'an

Today, I am going to Rome for a week, teaching my PhD course on ABC I first gave in Paris. The course takes place in La Sapienza Università di Roma, from Monday till Thursday. There will be an R lab in addition to the lectures. (I have no further item ... [Read more...]

Large-scale Inference

February 23, 2012 | xi'an

Large-scale Inference by Brad Efron is the first IMS Monograph in this new series, coordinated by David Cox and published by Cambridge University Press. Since I read this book immediately after Cox’ and Donnelly’s Principles of Applied Statistics, I was thinking of drawing a parallel between the two books. ... [Read more...]

another X’idated question

February 23, 2012 | xi'an

An X’idated reader of Monte Carlo Statistical Methods had trouble with our Example 3.13, the very one our academic book reviewer disliked so much as to “diverse [sic] a 2 star”. The issue is with computing the integral when f is the Student’s t(5) distribution density. In our book, we ... [Read more...]

weird [lack of] control…

February 20, 2012 | xi'an

When I ran I was expecting the same output as So this means that the dummy index in R “for” loops cannot be tweaked that easily. I seem to remember doing this kind of (dirty) tricks with earlier versions… Now, Alessandra and Robin think this is a good thing that ... [Read more...]

Cross validated question

February 19, 2012 | xi'an

Another problem generated by X’validated (on which I spent much too much time!): given an unbiased coin that produced M heads in the first M tosses, what is the expected number of additional tosses needed to get N (N__M) consecutive heads? Consider the preliminary question of getting a ... [Read more...]
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