A schedule that reinforces a complex operant consisting of completion of a schedule requirement (e.g., with FR 10 reinforced according to an Fl schedule, every tenth response that occurs at least 50 seconds after the last reinforcer is reinforced; in this example, FR 10 is the first-order schedule and Fl 50-s the second-order schedule). Such schedules often include a stimulus presented upon each completion of the first-order schedule, (e.g., adding a brief flash of light after every tenth response to the above example). The notation for such schedules includes the first-order schedule and the stimulus it produces in parentheses. Fl 50-s (FR 10. stimulus). A percentage-reinforcement schedule is a higher-order schedule in which the second-order schedule is a VR.