A variable interval is a schedule of reinforcement where a response is rewarded after an unpredictable amount of time has passed.
Variable Ratio Schedule: In this type, reinforcement comes after an unpredictable number of responses have been made by the subject.
Fixed Ratio Schedule: Here, reinforcement occurs after every nth (fixed) number of responses by the subject.
Random Reinforcement: This involves providing reinforcement at random intervals, making prediction impossible.