a schedule of reinforcement in which the units counted are not single responses but completions of a particular reinforcement schedule (the unit schedule). For example, in a second-order fixed-ratio 5 of fixed-interval 30-second schedule [FR 5 (FI 30 seconds)], reinforcement is delivered only after five successive FI 30-second schedules have been completed. Often, a brief stimulus of some sort is presented on completion of each unit schedule.