  We use statistical tools to characterize the response of an excitable system
to periodic perturbations. The system is an optically injected semiconductor
laser under pulsed perturbations of the phase of the injected field. We
characterize the laser response by counting the number of pulses emitted by the
laser, within a time interval, $\Delta$T , that starts when a perturbation is
applied. The success rate, SR($\Delta$T), is then defined as the number of
pulses emitted in the interval $\Delta$T , relative to the number of
perturbations. The analysis of the variation of SR with $\Delta$T allows to
separate a constant lag of technical origin and a frequency-dependent lag of
physical and dynamical origin. Once the lag is accounted for, the success rate
clearly captures locked and unlocked regimes and the transitions between them.
We anticipate that the success rate will be a practical tool for analyzing the
output of periodically forced systems, particularly when very regular
oscillations need to be generated via small periodic perturbations.
