## Roberto Casarin’s talk at CREST tomorrow

Posted in Statistics with tags , , , , , , , , , , , on March 13, 2019 by xi'an

My former student and friend Roberto Casarin (University Ca’Foscari, Venice) will talk tomorrow at the CREST Financial Econometrics seminar on

“Bayesian Markov Switching Tensor Regression for Time-varying Networks”

Time: 10:30
Date: 14 March 2019
Place: Room 3001, ENSAE, Université Paris-Saclay

Abstract : We propose a new Bayesian Markov switching regression model for multi-dimensional arrays (tensors) of binary time series. We assume a zero-inflated logit dynamics with time-varying parameters and apply it to multi-layer temporal networks. The original contribution is threefold. First, in order to avoid over-fitting we propose a parsimonious parameterisation of the model, based on a low-rank decomposition of the tensor of regression coefficients. Second, the parameters of the tensor model are driven by a hidden Markov chain, thus allowing for structural changes. The regimes are identified through prior constraints on the mixing probability of the zero-inflated model. Finally, we model the jointly dynamics of the network and of a set of variables of interest. We follow a Bayesian approach to inference, exploiting the Pólya-Gamma data augmentation scheme for logit models in order to provide an efficient Gibbs sampler for posterior approximation. We show the effectiveness of the sampler on simulated datasets of medium-big sizes, finally we apply the methodology to a real dataset of financial networks.

## finite mixture models [book review]

Posted in Books, Kids, Statistics, University life with tags , , , , , , , , , , , on February 17, 2014 by xi'an

Here is a review of Finite Mixture Models (2000) by Geoff McLachlan & David Peel that I wrote aeons ago (circa 1999), supposedly for JASA, which lost first the files and second the will to publish it. As I was working with my student today, I mentioned the book to her and decided to publish it here, if only because I think the book deserved a positive review, even after all those years! (Since then, Sylvia Frühwirth-Schnatter published Finite Mixture and Markov Switching Models (2004), which is closer to my perspective on the topic and that I would more naturally recommend.)

Mixture modeling, that is, the use of weighted sums of standard distributions as in

$\sum_{i=1}^k p_i f({\mathbf y};{\mathbf \theta}_i)\,,$

is a widespread and increasingly used technique to overcome the rigidity of standard parametric distributions such as f(y;θ), while retaining a parametric nature, as exposed in the introduction of my JASA review to Böhning’s (1998) book on non-parametric mixture estimation (Robert, 2000). This review pointed out that, while there are many books available on the topic of mixture estimation, the unsurpassed reference remained the book by Titterington, Smith and Makov (1985)  [hereafter TSM]. I also suggested that a new edition of TSM would be quite timely, given the methodological and computational advances that took place in the past 15 years: while it remains unclear whether or not this new edition will ever take place, the book by McLachlan and Peel gives an enjoyable and fairly exhaustive update on the topic, incorporating the most recent advances on mixtures and some related models.

Geoff McLachlan has been a major actor in the field for at least 25 years, through papers, software—the book concludes with a review of existing software—and books: McLachlan (1992), McLachlan and Basford (1988), and McLachlan and Krishnan (1997). I refer the reader to Lindsay (1989) for a review of the second book, which is a forerunner of, and has much in common with, the present book. Continue reading

## Catching up faster by switching sooner

Posted in R, Statistics, University life with tags , , , , , , , , on October 26, 2011 by xi'an

Here is our discussion (with Nicolas Chopin) of the Read Paper of last Wednesday by T. van Erven, P. Grünwald and S. de Rooij (Centrum voor Wiskunde en Informatica, Amsterdam), entitled Catching up faster by switching sooner: a predictive approach to adaptive estimation with an application to the Akaike information criterion–Bayesian information criterion dilemma. It is still available for written discussions, to be published in Series B. Even though the topic is quite tangential to our interests, the fact that the authors evolve in a Bayesian environment called for the following (my main contribution being in pointing out that the procedure is not Bayesian by failing to incorporate the switch in the predictive (6), hence using the same data for all models under competition…):

Figure 1 – Bayes factors of Model 2 vs.~Model 1 (gray line) and Model 3 vs.~Model 1 (dark line), plotted against the number of observations, i.e. of iterations, when comparing three stochastic volatility models; see Chopin et al. (2011) for full details.

This paper is an interesting attempt at a particularly important problem. We nonetheless believe more classical tools should be used instead if models are truly relevant in the inference led by the authors: Figure 1, reproduced from Chopin et al. (2011), plots [against time] the Bayes factors of Models 2 and 3 vs. Model 1, where all models are state-space models of increasing complexity, fitted to some real data. In this context, one often observes that more complex models need more time to “ascertain themselves”. On the other hand, even BMA based prediction is a very challenging computational problem (the only generic solution currently being the SMC² algorithm of the aforementioned paper), and we believe that the current proposed predictive strategy will remain too computationally expensive for practical use for nonlinear state-space models.

For other classes of models, since the provable methods put forward by this paper are based on “frozen strategies”, which are hard to defend from a modelling perspective, and since the more reasonable “basic switch” strategy seems to perform as well numerically, we would be curious to see how the proposed methods compare to predictive distributions obtained from genuine Bayesian models. A true change point model for instance would generate a coherent prediction strategy, which is not equivalent to the basic switch strategy. (Indeed, for one thing, the proposal made by the authors utilises the whole past to compute the switching probabilities, rather than allocating the proper portion of the data to the relevant model. In this sense, the proposal is “using the data [at least] twice” in a pseudo-Bayesian setting, similar to Aitkin’s, 1991.) More generally, the authors seem to focus on situations where the true generative process is a non-parametric class, and the completed models is an infinite sequence of richer and richer—but also of more and more complex—parametric models, which is a very sensible set-up in practice. Then, we wonder whether or not it would make more sense to set the prior distribution over the switch parameter s in such a way that (a) switches only occurs from one model to another model with greater complexity and (b) the number of switches is infinite.

For ABC readers, note the future Read Paper meeting on December 14 by Paul Fearnhead and Dennis Prangle.

## Hidden Markov mixtures of regression

Posted in Statistics with tags , , , , , on December 1, 2009 by xi'an

It took the RSS feed of Bayesian Analysis to disappear from my screen—because the Bayesian Analysis 4(4) issue was completed—for me to spot this very nice paper by Matthew A. Taddy and Athanasios Kottas on Markov switching regression models. It reminds me of earlier papers of mine’s with Monica Billio and Alain Monfort, and with Merrilee Hurn and Ana Justel, on Markov switching and mixtures of regression, respectively. At that time, with Merrilee, we had in mind to extend mixtures of regressions to generalised linear mixtures of generalised linear models but never found the opportunity to concretise the model. The current paper goes much farther by using mixtures of Dirichlet priors, thus giving a semi-parametric flavour to the mixture of regressions. There is also an interesting application to fishery management.

This issue also includes an emotional postnote by Brad Carlin, who is now stepping down from being the Bayesian Analysis Editor-in-chief. Brad unreservedly deserves thanks for mentoring Bayesian Analysis towards a wider audience and a stronger requirement on the papers being published in the journal. I think Bayesian Analysis now is a mainstream journal rather than the emanation of a society, albeit as exciting as ISBA! The electronic format adopted by Bayesian Analysis should be exploited further towards forums and on-line discussions of all papers, rather than singling out one paper by issue, and I am glad Brad agrees on this possible change of editorial policy. All the best to the new Editor-in-chief, Herbie Lee!