Research Discussion Paper – RDP 2025-03 Fast Posterior Sampling in Tightly Identified SVARs Using ‘Soft’ Sign Restrictions

We propose algorithms for conducting Bayesian inference in structural vector autoregressions identified using sign restrictions. The key feature of our approach is a sampling step based on ‘soft’ sign restrictions. This step draws from a target density that smoothly penalises parameter values violating the restrictions, facilitating the use of computationally efficient Markov chain Monte Carlo sampling algorithms. An importance-sampling step yields draws from the desired distribution conditional on the ‘hard’ sign restrictions. Relative to standard accept-reject sampling, the method substantially improves computational efficiency when identification is ‘tight’. It can also greatly reduce the computational burden of implementing prior-robust Bayesian methods. We illustrate the broad applicability of the approach in a model of the global oil market identified using a rich set of sign, elasticity and narrative restrictions.