Discrete state spaces represent a major computational challenge to statistical inference, since the computation of normalisation constants requires summation over large or possibly infinite sets, which can be impractical. This paper addresses this computational challenge through the development of a novel generalised Bayesian inference procedure suitable for discrete intractable likelihood. Inspired by recent methodological advances for continuous data, the main idea is to update beliefs about model parameters using a discrete Fisher divergence, in lieu of the problematic intractable likelihood. The result is a generalised posterior that can be sampled using standard computational tools, such as Markov chain Monte Carlo, circumventing the intractable normalising constant. The statistical properties of the generalised posterior are analysed, with sufficient conditions for posterior consistency and asymptotic normality established. In addition, a novel and general approach to calibration of generalised posteriors is proposed. Applications are presented on lattice models for discrete spatial data and on multivariate models for count data, where in each case the methodology facilitates generalised Bayesian inference at low computational cost.theoretical guarantees, and can be computed in a cost O(nd) that is linear in the size of the dataset. Full details of the DFD-Bayes approach are provided next.
MethodologyThis section presents and analyses DFD-Bayes. First, we present a novel discrete formulation of the Fisher divergence in Section 3.1. DFD-Bayes is introduced in Section 3.2, where posterior consistency and a Bernstein-von Mises result are established. Section 3.3 presents a novel approach to calibration of generalised posteriors, which may be of independent interest. Limitations of DFD-Bayes are discussed in Section 3.4.Notation Denote by X a countable set in which data are contained, and by Θ the set of permitted values for the parameter θ, where Θ is a Borel subset of R p for some p ∈ N. Probability distributions on X are identified with their probability mass functions, with respect to the counting measure on X . The i-th coordinate of a function f : X → R m is denoted by f i : X → R. For a probability distribution q on X and m, p ∈ N, denote by L p (q, R m ) the vector space of measurable functions f : X → R m such that m i=1 E X∼q [f i (X) p ] < ∞. For z ∈ R m , the Euclidean norm is denoted z . A Dirac measure at x ∈ X is denoted by δ x .