  We illustrate how the Hill relation and the notion of quasi-stationary
distribution can be used to analyse the biasing error introduced by many
numerical procedures that have been proposed in the literature, in particular
in molecular dynamics, to compute mean reaction times between metastable states
for Markov processes. The theoretical findings are illustrated on various
examples demonstrating the sharpness of the biasing error analysis as well as
the applicability of our study to elliptic diffusions.
