Differences between the RV posterior distribution as characterised through the MCMC methodology and the Laplace approximation for 27 ESPRESS018 observations of a K-type star (left column) and 21 ESPRESS019 observations of an M-type star (right column). Top panel: comparison of the RV expected value in m s⁻¹. We show, on each axis, the uncertainty associated with the corresponding measurement. Bottom: difference in the standard deviation of the RV posterior distribution between the two methodologies, in cm s⁻¹.