## a Nice talk

Posted in Books, Statistics, Travel, University life with tags , , , , , , , on February 20, 2015 by xi'an

Today, I give a talk on our testing paper in Nice, in a workshop run in connection with our Calibration ANR grant:

The slides are directly extracted from the paper but it still took me quite a while to translate the paper into those, during the early hours of our Czech break this week.

One added perk of travelling to Nice is the flight there, as it parallels the entire French Alps, a terrific view in nice weather!

## lazy ABC

Posted in Books, Statistics, University life with tags , , , , , , , on June 9, 2014 by xi'an

“A more automated approach would be useful for lazy versions of ABC SMC algorithms.”

Dennis Prangle just arXived the work on lazy ABC he had presented in Oxford at the i-like workshop a few weeks ago. The idea behind the paper is to cut down massively on the generation of pseudo-samples that are “too far” from the observed sample. This is formalised through a stopping rule that puts the estimated likelihood to zero with a probability 1-α(θ,x) and otherwise divide the original ABC estimate by α(θ,x). Which makes the modification unbiased when compared with basic ABC. The efficiency appears when α(θ,x) can be computed much faster than producing the entire pseudo-sample and its distance to the observed sample. When considering an approximation to the asymptotic variance of this modification, Dennis derives a optimal (in the sense of the effective sample size) if formal version of the acceptance probability α(θ,x), conditional on the choice of a “decision statistic” φ(θ,x).  And of an importance function g(θ). (I do not get his Remark 1 about the case when π(θ)/g(θ) only depends on φ(θ,x), since the later also depends on x. Unless one considers a multivariate φ which contains π(θ)/g(θ) itself as a component.) This approach requires to estimate

$\mathbb{P}(d(S(Y),S(y^o))<\epsilon|\varphi)$

as a function of φ: I would have thought (non-parametric) logistic regression a good candidate towards this estimation, but Dennis is rather critical of this solution.

I added the quote above as I find it somewhat ironical: at this stage, to enjoy laziness, the algorithm has first to go through a massive calibration stage, from the selection of the subsample [to be simulated before computing the acceptance probability α(θ,x)] to the construction of the (somewhat mysterious) decision statistic φ(θ,x) to the estimation of the terms composing the optimal α(θ,x). The most natural choice of φ(θ,x) seems to be involving subsampling, still with a wide range of possibilities and ensuing efficiencies. (The choice found in the application is somehow anticlimactic in this respect.) In most ABC applications, I would suggest using a quick & dirty approximation of the distribution of the summary statistic.

A slight point of perplexity about this “lazy” proposal, namely the static role of ε, which is impractical because not set in stone… As discussed several times here, the tolerance is a function of many factors incl. all the calibration parameters of the lazy ABC, rather than an absolute quantity. The paper is rather terse on this issue (see Section 4.2.2). It seems to me that playing with a large collection of tolerances may be too costly in this setting.

## the alive particle filter

Posted in Books, Statistics, University life with tags , , , , , , , , on February 14, 2014 by xi'an

As mentioned earlier on the ‘Og, this is a paper written by Ajay Jasra, Anthony Lee, Christopher Yau, and Xiaole Zhang that I missed when it got arXived (as I was also missing my thumb at the time…) The setting is a particle filtering one with a growing product of spaces and constraints on the moves between spaces. The motivating example is one of an ABC algorithm for an HMM where at each time, the simulated (pseudo-)observation is forced to remain within a given distance of the true observation. The (one?) problem with this implementation is that the particle filter may well die out by making only proposals that stand out of the above ball. Based on an idea of François Le Gland and Nadia Oudjane, the authors define the alive filter by imposing a fixed number of moves onto the subset, running a negative binomial number of proposals. By removing the very last accepted particle, they show that the negative binomial experiment allows for an unbiased estimator of the normalising constant(s). Most of the paper is dedicated to the theoretical study of this scheme, with results summarised as (p.2)

1. Time uniform Lp bounds for the particle filter estimates
2. A central limit theorem (CLT) for suitably normalized and centered particle filter estimates
3. An unbiased property of the particle filter estimates
4. The relative variance of the particle filter estimates, assuming N = O(n), is shown to grow linearly in n.

The assumptions necessary to reach those impressive properties are fairly heavy (or “exceptionally strong” in the words of the authors, p.5): the original and constrained transition kernels are both uniformly ergodic, with equivalent coverage of the constrained subsets for all possible values of the particle at the previous step. I also find the proposed implementation of the ABC filtering inadequate for approximating the posterior on the parameters of the (HMM) model. Expecting every realisation of the simulated times series to be close to the corresponding observed value is too hard a constraint. The above results are scaled in terms of the number N of accepted particles but it may well be that the number of generated particles and hence the overall computing time is much larger. In the examples completing the paper, the comparison is run with an earlier ABC sampler based on the very same stepwise proximity to the observed series so the impact of this choice is difficult to assess and, furthermore, the choice of the tolerances ε is difficult to calibrate: is 3, 6, or 12 a small or large value for ε? A last question that I heard from several sources during talks on that topic is why an ABC approach would be required in HMM settings where SMC applies. Given that the ABC reproduces a simulation on the pair latent variables x parameters, there does not seem to be a gain there…

## posterior predictive p-values

Posted in Books, Statistics, Travel, University life with tags , , , , , , , , , on February 4, 2014 by xi'an

Bayesian Data Analysis advocates in Chapter 6 using posterior predictive checks as a way of evaluating the fit of a potential model to the observed data. There is a no-nonsense feeling to it:

“If the model fits, then replicated data generated under the model should look similar to observed data. To put it another way, the observed data should look plausible under the posterior predictive distribution.”

And it aims at providing an answer to the frustrating (frustrating to me, at least) issue of Bayesian goodness-of-fit tests. There are however issues with the implementation, from deciding on which aspect of the data or of the model is to be examined, to the “use of the data twice” sin. Obviously, this is an exploratory tool with little decisional backup and it should be understood as a qualitative rather than quantitative assessment. As mentioned in my tutorial on Sunday (I wrote this post in Duke during O’Bayes 2013), it reminded me of Ratmann et al.’s ABCμ in that they both give reference distributions against which to calibrate the observed data. Most likely with a multidimensional representation. And the “use of the data twice” can be argued for or against, once a data-dependent loss function is built.

“One might worry about interpreting the significance levels of multiple tests or of tests chosen by inspection of the data (…) We do not make [a multiple test] adjustment, because we use predictive checks to see how particular aspects of the data would be expected to appear in replications. If we examine several test variables, we would not be surprised for some of them not to be fitted by the model-but if we are planning to apply the model, we might be interested in those aspects of the data that do not appear typical.”

The natural objection that having a multivariate measure of discrepancy runs into multiple testing is answered within the book with the reply that the idea is not to run formal tests. I still wonder how one should behave when faced with a vector of posterior predictive p-values (ppp).

The above picture is based on a normal mean/normal prior experiment I ran where the ratio prior-to-sampling variance increases from 100 to 10⁴. The ppp is based on the Bayes factor against a zero mean as a discrepancy. It thus grows away from zero very quickly and then levels up around 0.5, reaching only values close to 1 for very large values of x (i.e. never in practice). I find the graph interesting because if instead of the Bayes factor I use the marginal (numerator of the Bayes factor) then the picture is the exact opposite. Which, I presume, does not make a difference for Bayesian Data Analysis, since both extremes are considered as equally toxic… Still, still, still, we are is the same quandary as when using any kind of p-value: what is extreme? what is significant? Do we have again to select the dreaded 0.05?! To see how things are going, I then simulated the behaviour of the ppp under the “true” model for the pair (θ,x). And ended up with the histograms below:

which shows that under the true model the ppp does concentrate around .5 (surprisingly the range of ppp’s hardly exceeds .5 and I have no explanation for this). While the corresponding ppp does not necessarily pick any wrong model, discrepancies may be spotted by getting away from 0.5…

“The p-value is to the u-value as the posterior interval is to the confidence interval. Just as posterior intervals are not, in general, classical confidence intervals, Bayesian p-values are not generally u-values.”

Now, Bayesian Data Analysis also has this warning about ppp’s being not uniform under the true model (u-values), which is just as well considering the above example, but I cannot help wondering if the authors had intended a sort of subliminal message that they were not that far from uniform. And this brings back to the forefront the difficult interpretation of the numerical value of a ppp. That is, of its calibration. For evaluation of the fit of a model. Or for decision-making…

## discussione a Padova

Posted in Statistics, University life with tags , , , , , , , , , , , , on March 25, 2013 by xi'an

Here are the slides of my talk in Padova for the workshop Recent Advances in statistical inference: theory and case studies (very similar to the slides for the Varanasi and Gainesville meetings, obviously!, with Peter Müller commenting [at last!] that I had picked the wrong photos from Khajuraho!)

The worthy Padova addendum is that I had two discussants, Stefano Cabras from Universidad Carlos III in Madrid, whose slides are :

and Francesco Pauli, from Trieste, whose slides are:

These were kind and rich discussions with many interesting openings: Stefano’s idea of estimating the pivotal function h is opening new directions, obviously, as it indicates an additional degree of freedom in calibrating the method. Esp. when considering the high variability of the empirical likelihood fit depending on the the function h. For instance, one could start with a large collection of candidate functions and build a regression or a principal component reparameterisation from this collection… (Actually I did not get point #1 about ignoring f: the empirical likelihood is by essence ignoring anything outside the identifying equation, so as long as the equation is valid..) Point #2: Opposing sample free and simulation free techniques is another interesting venue, although I would not say ABC is “sample free”. As to point #3, I will certainly get a look at Monahan and Boos (1992) to see if this can drive the choice of a specific type of pseudo-likelihoods. I like the idea of checking the “coverage of posterior sets” and even more “the likelihood must be the density of a statistic, not necessarily sufficient” as it obviously relates with our current ABC model comparison work… Esp. when the very same paper is mentioned by Francesco as well. Grazie, Stefano! I also appreciate the survey made by Francesco on the consistency conditions, because I think this is an important issue that should be taken into consideration when designing ABC algorithms. (Just pointing out again that, in the theorem of Fearnhead and Prangle (2012) quoting Bernardo and Smith (1992), some conditions are missing for the mathematical consistency to apply.) I also like the agreement we seem to reach about ABC being evaluated per se rather than an a poor man’s Bayesian method. Francesco’s analysis of Monahan and Boos (1992) as validating or not empirical likelihood points out a possible link with the recent coverage analysis of Prangle et al., discussed on the ‘Og a few weeks ago. And an unsuspected link with Larry Wasserman! Grazie, Francesco!

## On optimality of kernels for ABC-SMC

Posted in Statistics, University life with tags , , , , , , , , , , on December 11, 2011 by xi'an

This freshly arXived paper by Sarah Filippi, Chris Barnes, Julien Cornebise, and Michael Stumpf, is in the lineage of our 2009 Biometrika ABC-PMC (population Monte Carlo) paper with Marc Beaumont, Jean-Marie Cornuet and Jean-Michel Marin. (I actually missed the first posting while in Berlin last summer. Flying to Utah gave me the opportunity to read it at length!)  The  paper focusses on the impact of the transition kernel in our PMC scheme: while we used component-wise adaptive proposals, the paper studies multivariate adaptivity with a covariance matrix adapted from the whole population, or locally or from an approximation to the information matrix. The simulation study run in the paper shows that, even when accounting for the additional cost due to the derivation of the matrix, the multivariate adaptation can improve the acceptance rate by a fair amount. So this is an interesting and positive sequel to our paper (that I may well end up refereeing one of those days, like an earlier paper from some of the authors!)

The main criticism I may have about the paper is that the selection of the tolerance sequence is not done in an adaptive way, while it could, given the recent developments of Del Moral et al. and of Drovandri and Pettitt (as well as our even more recent still-un-arXived submission to Stat & Computing!). While the target is the same for all transition kernels, thus the comparison still makes sense as is, the final product is to build a complete adaptive scheme that comes as close as possible to the genuine posterior.

This paper also raised a new question: there is a slight distinction between the Kullback-Leibler divergence we used and the Kullback-Leibler divergence the authors use here. (In fact, we do not account for the change in the tolerance.) Now, since what only matters is the distribution of the current particles, and while the distribution on the past particles is needed to compute the double integral leading to the divergence, there is a complete freedom in the choice of this past distribution. As in Del Moral et al., the distribution L(θ:t-1t) could therefore be chosen towards an optimal acceptance rate or something akin. I wonder if anyone ever looked at this…

## Semi-automatic ABC [discussion draft]

Posted in Statistics, Travel, University life with tags , , , , , , on November 3, 2011 by xi'an

Tomorrow, we hold a local seminar at CREST about the incoming Read Paper on December 14 by Paul Fearnhead and Dennis Prangle. I have already commented the paper in several posts (here and there), so here are my slides to summarise the paper and to introduce the discussion. I hope we can produce several submitted discussions out of this seminar!

(Warning: while I intend to attend the meeting next December 14 and to contribute a discussion, if not stranded by a snowstorm in the US!, the above is not the discussion I plan to present in front of the Society!)