## Conformal Bayesian Computation

**E**dwin Fong and Chris Holmes (Oxford) just wrote a paper on Bayesian scalable methods from a M-open perspective. Borrowing from the conformal prediction framework of Vovk et al. (2005) to achieve frequentist coverage for prediction intervals. The method starts with the choice of a conformity measure that measures how well each observation in the sample agrees with the sample. Which is exchangeable and hence leads to a rank statistic from which a p-value can be derived. Which is the empirical cdf associated with the observed conformities. Following Vovk et al. (2005) and Wasserman (2011) Edwin and Chris note that the Bayesian predictive itself acts like a conformity measure. Predictive that can itself be approximated by MCMC and importance sampling (possibly smoothed by Pareto). The paper also expands the setting to partial exchangeable models, renamed group conformal predictions. While reluctant to engage into turning Bayesian solutions into frequentist ones, I can see some worth in deriving both in order to expose discrepancies and hence signal possible issues with models and priors.

August 31, 2021 at 2:18 pm

The authors explain their motivation in Section 2.4, and signalling possible issues is prominent there. If the model or prior are misspecified, the Conformal Bayesian prediction intervals are likely to be much wider than the Bayesian ones. And the former might be a way of reconciling different opinions among Bayesians.