Which statement best describes a variable interval schedule?

• A. Rewards are delivered after a fixed number of correct responses.
• B. Rewards are delivered after a predictable time interval.
• C. Rewards are delivered after a random time interval.
• D. Rewards are delivered at unpredictable times, with the interval varying.

Answer: (D)

Variable interval reinforcement means rewards are given after an unpredictable amount of time, with the length of that interval changing from one reinforcement to the next. Because the next reward isn’t on a predictable clock, the dog keeps responding at a steady rate instead of concentrating behavior only near a known reward time. This consistency is useful in training because it promotes ongoing vigilance and steady performance. The other descriptions describe timing that’s fixed or tied to a set number of responses, which leads to different patterns of behavior; the key element here is unpredictability with varying intervals.