Which schedule reinforces after an amount of time has passed that varies unpredictably?

• A. Fixed Interval
• B. Fixed Duration
• C. Variable Duration
• D. Variable Interval

Answer: (D)

This item is about how timing of reinforcement shapes behavior when the wait for reinforcement is unpredictable. In a variable interval schedule, reinforcement is given for the first correct response after a time interval that changes from one trial to the next. Because the exact moment the reward will come is unknown, the subject tends to respond steadily and consistently rather than pausing or clustering responses around a predictable time point.

If the interval were fixed, you’d see a predictable pause after each reward followed by a burst of activity as the next interval nears completion. If the setup required holding or performing a behavior for a fixed duration, that would be a fixed duration schedule. If the required duration itself varied, that would be a variable duration schedule. Here, the key idea is that the reward timing depends on time passing, but the amount of time is unpredictable, which is why the reinforcement pattern is steady rather than timed to a specific moment.