## Bayes factors revisited

Posted in Books, Mountains, pictures, Statistics, Travel, University life with tags , , , , , , , , , on March 22, 2021 by xi'an

“Bayes factor analyses are highly sensitive to and crucially depend on prior assumptions about model parameters (…) Note that the dependency of Bayes factors on the prior goes beyond the dependency of the posterior on the prior. Importantly, for most interesting problems and models, Bayes factors cannot be computed analytically.”

Daniel J. Schad, Bruno Nicenboim, Paul-Christian Bürkner, Michael Betancourt, Shravan Vasishth have just arXived a massive document on the Bayes factor, worrying about the computation of this common tool, but also at the variability of decisions based on Bayes factors, e.g., stressing correctly that

“…we should not confuse inferences with decisions. Bayes factors provide inference on hypotheses. However, to obtain discrete decisions (…) from continuous inferences in a principled way requires utility functions. Common decision heuristics (e.g., using Bayes factor larger than 10 as a discovery threshold) do not provide a principled way to perform decisions, but are merely heuristic conventions.”

The text is long and at times meandering (at least in the sections I read), while trying a wee bit too hard to bring up the advantages of using Bayes factors versus frequentist or likelihood solutions. (The likelihood ratio being presented as a “frequentist” solution, which I think is an incorrect characterisation.) For instance, the starting point of preferring a model with a higher marginal likelihood is presented as an evidence (oops!) rather than argumented. Since this quantity depends on both the prior and the likelihood, it being high or low is impacted by both. One could then argue that using its numerical value as an absolute criterion amounts to selecting the prior a posteriori as much as checking the fit to the data! The paper also resorts to the Occam’s razor argument, which I wish we could omit, as it is a vague criterion, wide open to misappropriation. It is also qualitative, rather than quantitative, hence does not help in calibrating the Bayes factor.

Concerning the actual computation of the Bayes factor, an issue that has always been a concern and a research topic for me, the authors consider only two “very common methods”, the Savage–Dickey density ratio method and bridge sampling. We discussed the shortcomings of the Savage–Dickey density ratio method with Jean-Michel Marin about ten years ago. And while bridge sampling is an efficient approach when comparing models of the same dimension, I have reservations about this efficiency in other settings. Alternative approaches like importance nested sampling, noise contrasting estimation or SMC samplers are often performing quite efficiently as normalising constant approximations. (Not to mention our version of harmonic mean estimator with HPD support.)

Simulation-based inference is based on the notion that simulated data can be produced from the predictive distributions. Reminding me of ABC model choice to some extent. But I am uncertain this approach can be used to calibrate the decision procedure to select the most appropriate model. We thought about using this approach in our testing by mixture paper and it is favouring the more complex of the two models. This seems also to occur for the example behind Figure 5 in the paper.

Two other points: first, the paper does not consider the important issue with improper priors, which are not rigorously compatible with Bayes factors, as I discussed often in the past. And second, Bayes factors are not truly Bayesian decision procedures, since they remove the prior weights on the models, thus the mention of utility functions therein seems inappropriate unless a genuine utility function can be produced.

## sandwiching a marginal

Posted in Books, pictures, Statistics, University life with tags , , , , , , , , on March 8, 2021 by xi'an

When working recently on a paper for estimating the marginal likelihood, I was pointed out this earlier 2015 paper by Roger Grosse, Zoubin Ghahramani and Ryan Adams, which had escaped till now. The beginning of the paper discusses the shortcomings of importance sampling (when simulating from the prior) and harmonic mean (when simulating from the posterior) as solution. And of anNealed importance sampling (when simulating from a sequence, which sequence?!, of targets). The authors are ending up proposing a sequential Monte Carlo or (posterior) particle learning solution. A remark on annealed importance sampling is that there exist both a forward and a backward version for estimating the marginal likelihood, either starting from a simulation from the prior (easy) or from a simulation from the posterior (hard!). As in, e.g., Nicolas Chopin’s thesis, the intermediate steps are constructed from a subsample of the entire sample.

In this context, unbiasedness can be misleading: because partition function estimates can vary over many orders of magnitude, it’s common for an unbiased estimator to drastically underestimate Ζ with overwhelming probability, yet occasionally return extremely large estimates. (An extreme example is likelihood weighting, which is unbiased, but is extremely unlikely to give an accurate answer for a high-dimensional model.) Unless the estimator is chosen very carefully, the variance is likely to be extremely large, or even infinite.”

One novel aspect of the paper is to advocate for the simultaneous use of different methods and for producing both lower and upper bounds on the marginal p(y) and wait for them to get close enough. It is however delicate to find upper bounds, except when using the dreaded harmonic mean estimator.  (A nice trick associated with reverse annealed importance sampling is that the reverse chain can be simulated exactly from the posterior if associated with simulated data, except I am rather lost at the connection between the actual and simulated data.) In a sequential harmonic mean version, the authors also look at the dangers of using an harmonic mean but argue the potential infinite variance of the weights does not matter so much for log p(y), without displaying any variance calculation… The paper also contains a substantial experimental section that compares the different solutions evoked so far, plus others like nested sampling. Which did not work poorly in the experiment (see below) but could not be trusted to provide a lower or an upper bound. The computing time to achieve some level of agreement is however rather daunting. An interesting read definitely (and I wonder what happened to the paper in the end).

## approximation of Bayes Factors via mixing

Posted in Books, Statistics, University life with tags , , , , , , , , , , , on December 21, 2020 by xi'an

A [new version of a] paper by Chenguang Dai and Jun S. Liu got my attention when it appeared on arXiv yesterday. Due to its title which reminded me of a solution to the normalising constant approximation that we proposed in the 2010 nested sampling evaluation paper we wrote with Nicolas. Recovering bridge sampling—mentioned by Dai and Liu as an alternative to their approach rather than an early version—by a type of Charlie Geyer (1990-1994) trick. (The attached slides are taken from my MCMC graduate course, with a section on the approximation of Bayesian normalising constants I first wrote for a short course at Jim Berger’s 70th anniversary conference, in San Antonio.)

A difference with the current paper is that the authors “form a mixture distribution with an adjustable mixing parameter tuned through the Wang-Landau algorithm.” While we chose it by hand to achieve sampling from both components. The weight is updated by a simple (binary) Wang-Landau version, where the partition is determined by which component is simulated, ie by the mixture indicator auxiliary variable. Towards using both components on an even basis (à la Wang-Landau) and stabilising the resulting evaluation of the normalising constant. More generally, the strategy applies to a sequence of surrogate densities, which are chosen by variational approximations in the paper.

## nested sampling X check

Posted in Books, Mountains, pictures, Statistics with tags , , , , , , on September 18, 2020 by xi'an

Andrew Fowlie, Will Handley and Liangliang Su have recently arXived a new paper on checking the convergence of nested sampling by a uniformity test. The argument goes as follows: if the draw from the prior under the likelihood restriction (at the core of the nested sampling principle) is correctly generated, the rank of the realised value of the associated likelihood should be uniformly distributed among the remaining likelihoods. Obviously, the opposite does not hold:  a perfectly uniform distribution can happen even when the sampler misses a particularly well-hidden mode of the target disstribution or when it systematically stops too early, using for instance a misspecified bound on the likelihood. One particular setting when uniformity fails is when the likelihood surface plateaus in a particular region of the parameter space. (As a French speaker, writing plateaus makes me cringe since the plural of plateau is plateaux! Pardon my French!) When reaching the plateau the algorithm starts accumulating at the limiting value (or else completely ignores the plateau and its prior mass). I actually wonder if the existence of plateaux is not a sufficient reason for invalidating nested sampling, at least in its original version, since it assumes a continuous distribution on the likelihood values… If no plateau comes to hinder the algorithm, the rank test could be used to calibrate the exploration algorithm as for instance in the determination of the number of MCMC steps, running in parallel T random walks until the rank test across these runs turns green. The authors of the paper suggest using a Kolmogorov-Smirnov test, which strikes me as not the most appropriate solution, given the discrete nature of the theoretical distribution and the existence of uniformity tests in the pseudo random generation literature. At a conceptual level, I am also wondering at the sequential use of the test (as opposed to a parallel version at each iteration) since the target distribution is changing at every step (and so does the approximate method used to reproduce the prior simulation under the likelihood restriction).

## state of the art in sampling & clustering [workshop]

Posted in Books, pictures, Statistics, Travel, University life with tags , , , , , , , , , , on September 17, 2020 by xi'an

Next month, I am taking part in a workshop on sampling & clustering at the Max-Planck-Institut für Physik in Garching, Germany (near München). By giving a three hour introduction to ABC, as I did three years ago in Autrans. Being there and talking with local researchers if the sanitary conditions allow. From my office otherwise. Other speakers include Michael Betancourt on HMC and Johannes Buchner on nested sampling. The remote participation to this MPI workshop is both open and free, but participants must register before 18 September, namely tomorrow.