**J**ust another perfect day in Providence! After a brisk run in the eearly morning which took me through Brown campus, I attended the lecture by Sean Meyn on feedback particle filters. As it was mostly on diffusions with control terms, just too far from my field, I missed most of the points. (My fault, not Sean’s!) Then Ramon von Handel gave a talk about the curse(s) of dimensionality in particle filters, much closer to my interests, with a good summary of why (optimal) filters were not suffering from a curse in n, the horizon size, but in d, the dimension of the space, followed by an argument that some degree of correlation decay could overcome this dimensional curse as well. After the lunch break (where I thought further about the likelihood principle!), Dana Randall gave a technical talk on mixing properties of the hardcore model on Z² and bounding the cutoff parameter, which is when I appreciated the ability to follow talks from the ICERM lounge, watching slides and video of the talk taking place on the other side of the wall! At last, and in a programming contrapoint from slowly mixing to fastest mixing, Jim Fill presented his recent work on ordering Markov chains and finding fastest-mixing chains, which of course reminded me of Peskun ordering although there may be little connection in the end. The poster session in the evening had sufficiently few posters to make the discussion with each author enjoyable and relevant.A consistent feature of the meeting thus, allowing for quality interacting time between participants. I am now looking forward the final day with a most intriguing title by my friend Eric Moulines on TBA…

## Archive for November, 2012

## ICERM, Brown, Providence, RI (#2)

Posted in pictures, Running, Statistics, Travel, University life with tags Brown University, computational probability, computational statistics, ICERM, Providence, Rhode Island, simulation, workshop on November 30, 2012 by xi'an## the likelihood principle (sequel)

Posted in Statistics with tags ABC model choice, Allan Birnbaum, paradoxes, PNAS, sufficiency, The Likelihood Principle on November 30, 2012 by xi'an**A**s mentioned in my review of *Paradoxes in Scientific Inference* I was a bit confused by this presentation of the likelihood principle and this led me to ponder for a week or so whether or not there was an issue with Birnbaum’s proof (or, much more likely, with my vision of it!). After reading again Birnbaum’s proof, while sitting down in a quiet room at ICERM for a little while, I do not see any reason to doubt it. *(Keep reading at your own risk!)*

**M**y confusion was caused by mixing *sufficiency* in the sense of Birnbaum’s mixed experiment with *sufficiency* in the sense of our ABC model choice PNAS paper, namely that sufficient statistics are not always sufficient to select the right model. The sufficient statistics in the proof reduces the (2,x_{2}) observation from Model 2 to (1,x_{1}) from Model 1 when there is an observation x_{1 }that produces a likelihood proportional to the likelihood for x_{2} and the statistic is indeed sufficient: the distribution of (2,x_{2}) given (1,x_{1}) does not depend on the parameter θ. Of course, the statistic is not sufficient (most of the time) for deciding between Model 1 and Model 2, but this model choice issue is foreign to Birnbaum’s construction.

## ICERM, Brown, Providence, RI (#1)

Posted in Statistics, Travel, University life with tags Brown University, church, ICERM, Providence, Rhode Island on November 29, 2012 by xi'an**A**s I mentioned yesterday, and earlier, I was rather excited by the visit of the ICERM building. As it happens, the centre is located at the upper floor of a (rather bland!) 11 floor building sitting between Main St. and the river. It is quite impressive indeed, with a feeling of space due to the high ceilings and the glass walls all around the conference room, plus pockets of quietness with blackboards at the rescue. The whiteboard that makes the wall between the conference room and the lobby is also appreciable for discussion as it is huge (the whole wall is the whiteboard!) and made of a glassy material that makes writing on it a true pleasure (the next step would be to have a recording device embedded in it!). When I gave my talk and attended the other three talks of the day, I kind of regretted that the dual projector system would not allow for a lag of sorts in the presentation. Even though the pace of the other talks was quite reasonable (mine was a bit hurried I am afraid!), writing down a few notes was enough for me to miss some point from the previous slide. With huge walls, it should be easy to project at least the previous slide at the same time and maybe even all of the previous slide (maybe, maybe not, as it would get quickly confusing…)

**P**aul Dupuis’ talk covered new material (at least for me) on importance sampling for diffusions and the exploration of equilibriums, and it was thus quite enjoyable, even when fighting one of my dozing attacks. Gareth Roberts’ talk provided a very broad picture of the different optimal scalings (*à la 0.234!*) for MCMC algorithms (while I have attended several lectures by Gareth on this theme, there is always something new and interesting coming out of them!). Krzysztof Latuszynski’s talk on irreducible diffusions and the construction of importance sampling solutions replacing the (unavailable) exact sampling of Beskos et al. (2006) led to some discussion on the handling of negative weights. This is a question that has always intrigued me: if unbiasedness or exact simulation or something else induce negative weights in a sample, how can we process those weights when resampling? The conclusion of the discussion was that truncating the weights to zero seemed like the best solution, at least when resampling since the weights can be used as such in averages, but I wonder if there is a more elaborate scheme involving mixtures or whatnot!

## ICERM, Brown, Providence, RI (#0)

Posted in Running, Statistics, Travel, University life with tags ABC, Brown University, computational probability, computational statistics, ICERM, Providence, Rhodes Island, tea, workshop on November 28, 2012 by xi'an**I** have just arrived in Providence, RI, for the ICERM workshop on Performance Analysis of Monte Carlo Methods. While the plane trip was uneventful and even relaxing, as I could work on the revision to our ABCel (soon to be BCel!) paper, the bus trip from Boston to Providence, while smooth, quiet, wirelessed, and on-time, was a wee too much as it was already late for my standards… Anyway, I am giving one of the talks tomorrow, with a *pot-pourri* on ABC and empirical likelihood as in Ames and Chicago last month. The format of the workshop sounds very nice, with only four talks a day, which should leave a lot of space for interactions between participants (if I do not crash from my early early rise…) And, as mentioned earlier, I am looking forward visiting the futuristic building.