A variable-interval schedule delivers reinforcement for the first response after a varying amount of time has passed. The interval changes unpredictably from one reinforcer to the next.

Because the next opportunity for reinforcement could occur at any moment, responding tends to be steady and moderate. Checking email is often cited as a real-world approximation: messages arrive at irregular intervals, and each check is reinforced only when a message is present. In the laboratory, a variable-interval schedule produces a stable response rate without the long pauses characteristic of fixed-interval schedules.

Behavior maintained by variable-interval reinforcement is relatively resistant to extinction. The unpredictability of the interval makes the absence of reinforcement less informative, so responding continues longer after reinforcement stops. The schedule is useful when a consistent, ongoing rate of behavior is desired without the scalloped pattern of fixed intervals.

When reinforcement becomes available after unpredictable waits, responding settles into a steady rhythm.