[latexpage]This post gives a brief introduction to the pseudo-marginal approach to MCMC. A very nice explanation, with examples, is available here. Frequently, we are given a density function $\pi({\bf x})$, with ${\bf x} \in \mathcal X$, and we use Markov chain Monte Carlo (MCMC) to generate samples from the corresponding probability distribution. For simplicity, suppose we are performing Metropolis-Hastings with a spherical proposal distribution. Then, we move from the current state ${\bf x}$ to a proposed state ${\bf x}’$ with probability $\min(1, \pi({\bf x}’)/\pi({\bf x}))$ .

But what if we cannot evaluate $\pi({\bf x})$ exactly? Such a situation might arise if we are given a joint density function $\pi({\bf x}, {\bf z})$, with ${\bf z} \in \mathcal Z$, and we must marginalize out ${\bf z}$ in order to compute $\pi({\bf x})$. In this situation, we may only be able to approximate

\[ \pi({\bf x}) = \int \pi({\bf x},{\bf z}) \, \mathrm d{\bf z} ,\] Continue reading “Pseudo-marginal MCMC”