A fixed-interval schedule delivers reinforcement for the first response that occurs after a fixed amount of time has passed. The interval is constant from one reinforcement to the next.

Behavior under this schedule shows a characteristic scalloped pattern. Responding is slow or absent just after reinforcement, then accelerates as the end of the interval approaches. A student who studies little until just before a weekly quiz, or a worker who becomes more productive as payday nears, is displaying the same pattern. The organism has learned roughly when the next reinforcer will become available and concentrates effort near that time.

Fixed-interval schedules produce less total responding than ratio schedules and are relatively easy to extinguish once reinforcement stops. They are common in everyday life whenever consequences become available on a clock or calendar rather than after a certain number of actions.

When the reinforcer is tied to the passage of time, responding waits for the clock.