Which schedule changes the time interval required for reinforcement in an unpredictable way?

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

Answer: (B)

The key idea is how reinforcement timing is determined by the schedule. If the requirement to earn reinforcement itself changes from trial to trial, the time you must wait or the duration you must perform the behavior before getting reinforced becomes unpredictable.

Variable Duration does exactly that: the length of time the subject must maintain or perform the behavior before reinforcement is delivered varies across trials. Because that duration changes, the time interval to reinforcement is unpredictable, which is what the question is asking about.

In contrast, fixed interval creates a predictable waiting period before reinforcement after a response; fixed duration has a fixed time requirement to obtain reinforcement once the behavior starts; and variable interval makes the time between reinforcement opportunities unpredictable, but it doesn’t change the duration of the required behavior itself.