Step 1: Understanding reinforcement schedules.
Reinforcement schedules describe how and when rewards are delivered following a behavior. Step 2: Variable-interval schedule.
In a variable-interval schedule, reinforcement is given after unpredictable or varying time intervals, regardless of the number of responses. Step 3: Distinction from other schedules.
Fixed schedules use constant time or response counts, while ratio schedules depend on number of responses rather than time. Step 4: Conclusion.
Thus, reinforcement based on varying amounts of time is called a variable-interval schedule.