  We reconsider the theory of the linear response of non-equilibrium steady
states to perturbations. We first show that by using a general functional
decomposition for space-time dependent forcings, we can define elementary
susceptibilities that allow to construct the response of the system to general
perturbations. Starting from the definition of SRB measure, we then study the
consequence of taking different sampling schemes for analysing the response of
the system. We show that only a specific choice of the time horizon for
evaluating the response of the system to a general time-dependent perturbation
allows to obtain the formula first presented by Ruelle. We also discuss the
special case of periodic perturbations, showing that when they are taken into
consideration the sampling can be fine-tuned to make the definition of the
correct time horizon immaterial. Finally, we discuss the implications of our
results in terms of strategies for analyzing the outputs of numerical
experiments by providing a critical review of a formula proposed by Reick.
