A schedule in which some minimum time must elapse before a response is reinforced; early responses have no effect. The time is measured from some event, typically a stimulus on set or the last reinforcer (an alternate method times each interval from the end of the last one, without regard to the time from the end of the interval to the reinforced response). In fixed-interval (FI schedules, the time is constant from one interval to the next, and performance is characterized by a pause after the reinforcer followed by a gradual or an abrupt transition to a moderate rate of responding. In variable-interval (VI) schedules, the time varies from one reinforcer to the next; compared with Fl schedules, the rate of responding is relatively constant between reinforcers. Interval schedules are usually identified by average interval (e.g., FI 60-s and VI 60-s arrange one reinforcer per minute).