Importance Nested Sampling and the MultiNest Algorithm

TL;DR

This paper introduces importance nested sampling (INS), an enhancement of the MultiNest algorithm, which significantly improves the accuracy of Bayesian evidence estimation in complex, multi-modal Bayesian inference problems.

Contribution

The paper presents INS as an alternative summation method for MultiNest, achieving up to tenfold higher accuracy in Bayesian evidence calculation without altering the sampling process.

Findings

INS outperforms vanilla NS in evidence accuracy

INS can handle highly multi-modal posteriors effectively

Application to test problems demonstrates improved precision

Abstract

Bayesian inference involves two main computational challenges. First, in estimating the parameters of some model for the data, the posterior distribution may well be highly multi-modal: a regime in which the convergence to stationarity of traditional Markov Chain Monte Carlo (MCMC) techniques becomes incredibly slow. Second, in selecting between a set of competing models the necessary estimation of the Bayesian evidence for each is, by definition, a (possibly high-dimensional) integration over the entire parameter space; again this can be a daunting computational task, although new Monte Carlo (MC) integration algorithms offer solutions of ever increasing efficiency. Nested sampling (NS) is one such contemporary MC strategy targeted at calculation of the Bayesian evidence, but which also enables posterior inference as a by-product, thereby allowing simultaneous parameter estimation and model selection. The widely-used MultiNest algorithm presents a particularly efficient implementation of the NS technique for multi-modal posteriors. In this paper we discuss importance nested sampling (INS), an alternative summation of the MultiNest draws, which can calculate the Bayesian evidence at up to an order of magnitude higher accuracy than `vanilla' NS with no change in the way MultiNest explores the parameter space. This is accomplished by treating as a (pseudo-)importance sample the totality of points collected by MultiNest, including those previously discarded under the constrained likelihood sampling of the NS algorithm. We apply this technique to several challenging test problems and compare the accuracy of Bayesian evidences obtained with INS against those from vanilla NS.