In a variable-ratio schedule, when is reinforcement delivered?

In a variable-ratio schedule, reinforcement is delivered after a random number of responses have been made. This means that the exact number of responses required for reinforcement varies from trial to trial, creating a high and steady rate of responding. This unpredictability encourages the individual or animal to continue engaging in the behavior, as they cannot predict when the next reinforcement will come, which often leads to greater persistence in the behavior.

The variability in the number of responses necessary for reinforcement distinguishes this schedule from others, such as fixed-ratio or fixed-interval schedules, where the requirements are consistent and predictable. This random aspect is integral to the effectiveness of the variable-ratio schedule in strengthening behavior.