Which schedule produces reinforcement after an unpredictable amount of time has elapsed?

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

Answer: (D)

Reinforcement after an unpredictable amount of time has elapsed is produced by a variable interval schedule. In this setup, the time between opportunities for reinforcement varies, so the subject cannot predict when reinforcement will become available. As a result, the organism tends to respond at a steady rate to maximize chances of getting reinforced, rather than slowing down or piling up responses as a deadline approaches.

To contrast, a fixed interval schedule delivers reinforcement after a set, predictable time has passed, which often leads to a post-reinforcement pause followed by increasing responding as the interval nears its end. A fixed duration schedule requires continuous behavior for a specific amount of time to obtain reinforcement, not about the timing between reinforcement opportunities. A variable duration schedule involves varying how long the behavior must be performed to earn reinforcement, rather than varying the time between reinforcements.