## Bayesian empirical likelihood

Posted in Books, pictures, Statistics with tags , , , , , , on July 21, 2016 by xi'an

Sid Chib, Minchul Shin, and Anna Simoni (CREST) recently arXived a paper entitled “Bayesian Empirical Likelihood Estimation and Comparison of Moment Condition Models“. That Sid mentioned to me in Sardinia. The core notion is related to earlier Bayesian forays into empirical likelihood pseudo-models, like Lazar (2005) or our PNAS paper with Kerrie Mengersen and Pierre Pudlo. Namely to build a pseudo-likelihood using empirical likelihood principles and to derive the posterior associated with this pseudo-likelihood. Some novel aspects are the introduction of tolerance (nuisance) extra-parameters when some constraints do not hold, a maximum entropy (or exponentially tilted) representation of the empirical  likelihood function, and a Chib-Jeliazkov representation of the marginal likelihood. The authors obtain a Bernstein-von Mises theorem under correct specification. Meaning convergence. And another one under misspecification.

While the above Bernstein-von Mises theory is somewhat expected (if worth deriving) in the light of frequentist consistency results, the paper also considers a novel and exciting aspect, namely to compare models (or rather moment restrictions) by Bayes factors derived from empirical likelihoods. A grand (encompassing) model is obtained by considering all moment restrictions at once, which first sounds like more restricted, except that the extra-parameters are there to monitor constraints that actually hold. It is unclear from my cursory read of the paper whether priors on those extra-parameters can be automatically derived from a single prior. And how much they impact the value of the Bayes factor. The consistency results found in the paper do not seem to depend on the form of priors adopted for each model (for all three cases of both correctly, one correctly and none correctly specified models). Except maybe for some local asymptotic normality (LAN). Interestingly (?), the authors consider the Poisson versus Negative Binomial test we used in our testing by mixture paper. This paper is thus bringing a better view of the theoretical properties of a pseudo-Bayesian approach based on moment conditions and empirical likelihood approximations. Without a clear vision of the implementation details, from the parameterisation of the constraints (which could be tested the same way) to the construction of the prior(s) to the handling of MCMC difficulties in realistic models.

## importance sampling schemes for evidence approximation [revised]

Posted in Statistics, University life with tags , , , , , , , on November 18, 2014 by xi'an

After a rather intense period of new simulations and versions, Juong Een (Kate) Lee and I have now resubmitted our paper on (some) importance sampling schemes for evidence approximation in mixture models to Bayesian Analysis. There is no fundamental change in the new version but rather a more detailed description of what those importance schemes mean in practice. The original idea in the paper is to improve upon the Rao-Blackwellisation solution proposed by Berkoff et al. (2002) and later by Marin et al. (2005) to avoid the impact of label switching on Chib’s formula. The Rao-Blackwellisation consists in averaging over all permutations of the labels while the improvement relies on the elimination of useless permutations, namely those that produce a negligible conditional density in Chib’s (candidate’s) formula. While the improvement implies truncated the overall sum and hence induces a potential bias (which was the concern of one referee), the determination of the irrelevant permutations after relabelling next to a single mode does not appear to cause any bias, while reducing the computational overload. Referees also made us aware of many recent proposals that conduct to different evidence approximations, albeit not directly related with our purpose. (One was Rodrigues and Walker, 2014, discussed and commented in a recent post.)