Archive for the University life Category

new version of abcrf

Posted in R, Statistics, University life with tags , , , , , , on February 12, 2016 by xi'an
fig-tree near Brisbane, Australia, Aug. 18, 2012Version 1.1 of our R library abcrf version 1.1  is now available on CRAN.  Improvements against the earlier version are numerous and substantial. In particular,  calculations of the random forests have been parallelised and, for machines with multiple cores, the computing gain can be enormous. (The package does along with the random forest model choice paper published in Bioinformatics.)

conference deadlines [register now!!]

Posted in Kids, pictures, Statistics, Travel, University life with tags , , , , , , , , , , on February 11, 2016 by xi'an

bike trail from Kenilworth to the University of WarwickRegistration is now open for our [fabulous!] CRiSM workshop on estimating [normalising] constants, in Warwick, on April 20-22 this year. While it is almost free (almost as in £40.00!), we strongly suggest you register asap if only to secure a bedroom on the campus at a moderate rate of £55.00 per night (breakfast included!). Plus we would like to organise the poster session(s) and the associated “elevator” talks for the poster presenters.

While the deadline for early registration at AISTATS is now truly over, we also encourage all researchers interested in this [great] conference to register as early as possible, if only [again] to secure a room at the conference location, the Parador Hotel in Cádiz. (Otherwise, there are plenty of rentals in the neighbourhood.)

Last and not least, the early registration for ISBA 2016 in Santa Margherita di Pula, Sardinia, is still open till February 29. And the rate will move immediately to late registration fees. The same deadline applies to bedroom reservations in the resort, with apparently only a few rooms left for some of the nights. Rentals and hotels around are also getting filled rather quickly.

Bayesian composite likelihood

Posted in Books, Statistics, University life with tags , , , , , , on February 11, 2016 by xi'an

“…the pre-determined weights assigned to the different associations between observed and unobserved values represent strong a priori knowledge regarding the informativeness of clues. A poor choice of weights will inevitably result in a poor approximation to the “true” Bayesian posterior…”

Last Xmas, Alexis Roche arXived a paper on Bayesian inference via composite likelihood. I find the paper quite interesting in that [and only in that] it defends the innovative notion of writing a composite likelihood as a pool of opinions about some features of the data. Recall that each term in the composite likelihood is a marginal likelihood for some projection z=f(y) of the data y. As in ABC settings, although it is rare to derive closed-form expressions for those marginals. The composite likelihood is parameterised by powers of those components. Each component is associated with an expert, whose weight reflects the importance. The sum of the powers is constrained to be equal to one, even though I do not understand why the dimensions of the projections play no role in this constraint. Simplicity is advanced as an argument, which sounds rather weak… Even though this may be infeasible in any realistic problem, it would be more coherent to see the weights as producing the best Kullback approximation to the true posterior. Or to use a prior on the weights and estimate them along the parameter θ. The former could be incorporated into the later following the approach of Holmes & Walker (2013). While the ensuing discussion is most interesting, it remains missing in connecting the different components in terms of the (joint) information brought about the parameters. Especially because the weights are assumed to be given rather than inferred. Especially when they depend on θ. I also wonder why the variational Bayes interpretation is not exploited any further. And see no clear way to exploit this perspective in an ABC environment.

snapshot from Oxford [#2]

Posted in Kids, pictures, Travel, University life with tags , , , , on February 9, 2016 by xi'an

covariant priors, Jeffreys and paradoxes

Posted in Books, Statistics, University life with tags , , , , , , , , , , , on February 9, 2016 by xi'an

“If no information is available, π(α|M) must not deliver information about α.”

In a recent arXival apparently submitted to Bayesian Analysis, Giovanni Mana and Carlo Palmisano discuss of the choice of priors in metrology. Which reminded me of this meeting I attended at the Bureau des Poids et Mesures in Sèvres where similar debates took place, albeit being led by ferocious anti-Bayesians! Their reference prior appears to be the Jeffreys prior, because of its reparameterisation invariance.

“The relevance of the Jeffreys rule in metrology and in expressing uncertainties in measurements resides in the metric invariance.”

This, along with a second order approximation to the Kullback-Leibler divergence, is indeed one reason for advocating the use of a Jeffreys prior. I at first found it surprising that the (usually improper) prior is used in a marginal likelihood, as it cannot be normalised. A source of much debate [and of our alternative proposal].

“To make a meaningful posterior distribution and uncertainty assessment, the prior density must be covariant; that is, the prior distributions of different parameterizations must be obtained by transformations of variables. Furthermore, it is necessary that the prior densities are proper.”

The above quote is quite interesting both in that the notion of covariant is used rather than invariant or equivariant. And in that properness is indicated as a requirement. (Even more surprising is the noun associated with covariant, since it clashes with the usual notion of covariance!) They conclude that the marginal associated with an improper prior is null because the normalising constant of the prior is infinite.

“…the posterior probability of a selected model must not be null; therefore, improper priors are not allowed.”

Maybe not so surprisingly given this stance on improper priors, the authors cover a collection of “paradoxes” in their final and longest section: most of which makes little sense to me. First, they point out that the reference priors of Berger, Bernardo and Sun (2015) are not invariant, but this should not come as a surprise given that they focus on parameters of interest versus nuisance parameters. The second issue pointed out by the authors is that under Jeffreys’ prior, the posterior distribution of a given normal mean for n observations is a t with n degrees of freedom while it is a t with n-1 degrees of freedom from a frequentist perspective. This is not such a paradox since both distributions work in different spaces. Further, unless I am confused, this is one of the marginalisation paradoxes, which more straightforward explanation is that marginalisation is not meaningful for improper priors. A third paradox relates to a contingency table with a large number of cells, in that the posterior mean of a cell probability goes as the number of cells goes to infinity. (In this case, Jeffreys’ prior is proper.) Again not much of a bummer, there is simply not enough information in the data when faced with a infinite number of parameters. Paradox #4 is the Stein paradox, when estimating the squared norm of a normal mean. Jeffreys’ prior then leads to a constant bias that increases with the dimension of the vector. Definitely a bad point for Jeffreys’ prior, except that there is no Bayes estimator in such a case, the Bayes risk being infinite. Using a renormalised loss function solves the issue, rather than introducing as in the paper uniform priors on intervals, which require hyperpriors without being particularly compelling. The fifth paradox is the Neyman-Scott problem, with again the Jeffreys prior the culprit since the estimator of the variance is inconsistent. By a multiplicative factor of 2. Another stone in Jeffreys’ garden [of forking paths!]. The authors consider that the prior gives zero weight to any interval not containing zero, as if it was a proper probability distribution. And “solve” the problem by avoid zero altogether, which requires of course to specify a lower bound on the variance. And then introducing another (improper) Jeffreys prior on that bound… The last and final paradox mentioned in this paper is one of the marginalisation paradoxes, with a bizarre explanation that since the mean and variance μ and σ are not independent a posteriori, “the information delivered by x̄ should not be neglected”.

Bayesian model comparison with intractable constants

Posted in Books, Kids, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , on February 8, 2016 by xi'an

abcIRichard Everitt, Adam Johansen (Warwick), Ellen Rowing and Melina Evdemon-Hogan have updated [on arXiv] a survey paper on the computation of Bayes factors in the presence of intractable normalising constants. Apparently destined for Statistics and Computing when considering the style. A great entry, in particular for those attending the CRiSM workshop Estimating Constants in a few months!

A question that came to me from reading the introduction to the paper is why a method like Møller et al.’s (2006) auxiliary variable trick should be considered more “exact” than the pseudo-marginal approach of Andrieu and Roberts (2009) since the later can equally be seen as an auxiliary variable approach. The answer was on the next page (!) as it is indeed a special case of Andrieu and Roberts (2009). Murray et al. (2006) also belongs to this group with a product-type importance sampling estimator, based on a sequence of tempered intermediaries… As noted by the authors, there is a whole spectrum of related methods in this area, some of which qualify as exact-approximate, inexact approximate and noisy versions.

Their main argument is to support importance sampling as the method of choice, including sequential Monte Carlo (SMC) for large dimensional parameters. The auxiliary variable of Møller et al.’s (2006) is then part of the importance scheme. In the first toy example, a Poisson is opposed to a Geometric distribution, as in our ABC model choice papers, for which a multiple auxiliary variable approach dominates both ABC and Simon Wood’s synthetic likelihood for a given computing cost. I did not spot which artificial choice was made for the Z(θ)’s in both models, since the constants are entirely known in those densities. A very interesting section of the paper is when envisioning biased approximations to the intractable density. If only because the importance weights are most often biased due to the renormalisation (possibly by resampling). And because the variance derivations are then intractable as well. However, due to this intractability, the paper can only approach the impact of those approximations via empirical experiments. This leads however to the interrogation on how to evaluate the validity of the approximation in settings where truth and even its magnitude are unknown… Cross-validation and bootstrap type evaluations may prove too costly in realistic problems. Using biased solutions thus mostly remains an open problem in my opinion.

The SMC part in the paper is equally interesting if only because it focuses on the data thinning idea studied by Chopin (2002) and many other papers in the recent years. This made me wonder why an alternative relying on a sequence of approximations to the target with tractable normalising constants could not be considered. A whole sequence of auxiliary variable completions sounds highly demanding in terms of computing budget and also requires a corresponding sequence of calibrations. (Now, ABC fares no better since it requires heavy simulations and repeated calibrations, while further exhibiting a damning missing link with the target density. ) Unfortunately, embarking upon a theoretical exploration of the properties of approximate SMC is quite difficult, as shown by the strong assumptions made in the paper to bound the total variation distance to the true target.

It’s the selection’s fault not the p-values’… [seminar]

Posted in pictures, Statistics, University life with tags , , , , , , on February 5, 2016 by xi'an

Paris and la Seine, from Pont du Garigliano, Oct. 20, 2011Yoav Benjamini will give a seminar talk in Paris next Monday on the above (full title: “The replicability crisis in science: It’s the selection’s fault not the p-values’“). (That I will miss for being in Warwick at the time.) With a fairly terse abstract:

I shall discuss the problem of lack of replicability of results in science, and point at selective inference as a statistical root cause. I shall then present a few strategies for addressing selective inference, and their application in genomics, brain research and earlier phases of clinical trials where both primary and secondary endpoints are being used.

Details: February 8, 2016, 16h, Université Pierre & Marie Curie, campus Jussieu, salle 15-16-101.


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