The two basic classes of intermittent reinforcement schedules are based on the number of responses and the time elapsed. Which pair correctly names these classes?

• A. Fixed ratio and variable interval
• B. Interval and ratio
• C. Continuous reinforcement
• D. Ratio and Interval

Answer: (D)

Intermittent reinforcement schedules are defined by what controls reinforcement: the number of responses or the passage of time. When reinforcement depends on a certain number of responses, it’s a ratio schedule. When it depends on time elapsed, it’s an interval schedule. The option that lists both broad categories directly—ratio and interval—matches this fundamental distinction. The other choices mix specific subtypes with these classes, point to continuous reinforcement (not intermittent), or just swap the order of the two terms. So, ratio and interval is the correct pairing.