Archive for the University life Category

ARS: when to update?

Posted in Books, Kids, Statistics, University life with tags , , , , , on May 25, 2017 by xi'an

An email I got today from Heng Zhou wondered about the validity of the above form of the ARS algorithm. As printed in our book Monte Carlo Statistical Methods. The worry is that in the original version of the algorithm the envelope of the log-concave target f(.) is only updated for rejected values. My reply to the question is that there is no difference in the versions towards returning a value simulated from f, since changing the envelope between simulations does not modify the accept-reject nature of the algorithm. There is no issue of dependence between the simulations of this adaptive accept-reject method, all simulations remain independent. The question is rather one about efficiency, namely does it pay to update the envelope(s) when accepting a new value and I think it does because the costly part is the computation of f(x), rather than the call to the piecewise-exponential envelope. Correct me if I am wrong!

end of a long era [1982-2017]

Posted in Books, pictures, Running, University life with tags , , , , , , , , , , , on May 23, 2017 by xi'an

This afternoon I went to CREST to empty my office there from books and a few papers (like the original manuscript version of Monte Carlo Statistical Methods). This is because the research centre, along with the ENSAE graduate school (my Alma mater), is moving to a new building on the Saclay plateau, next to École Polytechnique. As part of this ambitious migration of engineering schools from downtown Paris to a brand new campus there. Without getting sentimental about this move, it means leaving the INSEE building in Malakoff, on the outskirts of downtown Paris, which has been an enjoyable part of my student and then academic life from 1982 till now. And also leaving the INSEE Paris Club runners! (I am quite uncertain about being as active at the new location, if only because going there by bike is a bit more of a challenge. To be addressed anyway!) And I left behind my accumulation of conference badges (although I should try to recycle them for the incoming BNP 11 in Paris!).

Bacon in the Library [jatp]

Posted in Books, Kids, pictures, Travel, University life with tags , , , , , , , on May 23, 2017 by xi'an

ABCπ

Posted in Books, pictures, Statistics, Travel, University life with tags , , , , on May 17, 2017 by xi'an

Ritabrata Dutta, Marcel Schöengens, Jukka-Pekka Onnela, and Antonietta Mira recently put a new ABC software on-line, called ABCpy for ABC with Python. The software aims at  an automated parallelisation of ABC runs, requiring only code to generate from the (generative) model and the choice of summary statistics and of associated distance. Alternatively an approximate likelihood (as in synthetic likelihood) can be used. The tolerance ε is chosen as a percentile of the prior predictive distribution on the distance. The versions of ABC found in ABCpy are

  1. Population Monte Carlo for ABC (PMCABC);
  2. sequential Monte Carlo ABC (ABC-SMC);
  3. replenishment Sequential Monte Carlo ABC (RSMC-ABC);
  4. adaptive Population Monte Carlo ABC (APMCABC);
  5. ABC with subset simulation (ABCsubsim); and
  6. simulated annealing ABC (SABC)

Anto mentioned ABCpy to me while in Harvard last week and I have not tested the program (my only brush with Python being the occasional call to latex2wp for SeriesB’log). And obviously, writing a blog about Monte (Carlo and) Python makes a link to the Monty Pythons irresistible:

Dutch book for sleeping beauty

Posted in Books, Kids, Statistics, University life with tags , , , , , , , , on May 15, 2017 by xi'an

After my short foray in Dutch book arguments two weeks ago in Harvard, I spotted a recent arXival by Vincent Conitzer analysing the sleeping beauty paradox from a Dutch book perspective. (The paper “A Dutch book against sleeping beauties who are evidential decision theorists” actually appeared in Synthese two years ago, which makes me wonder why it comes out only now on arXiv. And yes I am aware the above picture is about Bansky’s Cindirella and not sleeping beauty!)

“if Beauty is an evidential decision theorist, then in variants where she does not always have the same information available to her upon waking, she is vulnerable to Dutch books, regardless of whether she is a halfer or a thirder.”

As recalled in the introduction of the paper, there exist ways to construct Dutch book arguments against thirders and halfers alike. Conitzer constructs a variant that also distinguishes between a causal and an evidential decision theorist (sleeping beauty), the later being susceptible to another Dutch book. Which is where I get lost as I have no idea of a distinction between those two types of decision theory. Quickly checking on Wikipedia returned the notion that the latter decision theory maximises the expected utility conditional on the decision, but this does not clarify the issue in that it seems to imply the decision impacts the probability of the event… Hence keeping me unable to judge of the relevance of the arguments therein (which is no surprise since only based on a cursory read).

Sequential Monte Carlo workshop in Uppsala

Posted in pictures, Statistics, Travel, University life with tags , , , , on May 12, 2017 by xi'an

A workshop on sequential Monte Carlo will take place in Uppsala, Sweeden, on August 30 – September 1, 2017. Involving 21 major players in the field. It follows SMC 2015 that took place at CREST and was organised by Nicolas Chopin. Furthermore, this workshop is preceded by a week-long PhD level course. (The above picture serves as background for the announcement and was taken by Lawrence Murray, whose multiple talents include photography.)

Alan Gelfand in Paris

Posted in pictures, Statistics, Travel, University life with tags , , , , , , , on May 11, 2017 by xi'an

Alan Gelfand (Duke University) will be in Paris on the week of May 15 and give several seminars, including one at AgroParisTech on May 16:

Modèles hiérarchiques

and on at CREST (BiPS)  on May 18, 2pm:

Scalable Gaussian processes for analyzing space and space-time datasets