Which schedule of reinforcement describes a reinforcer delivered contingent upon the first behavior following a variable time period?

The schedule of reinforcement that involves delivering a reinforcer contingent upon the first behavior following a variable time period is known as a Variable Interval schedule. This type of reinforcement schedule is characterized by the fact that the time intervals between reinforcements are unpredictable and vary around a certain average.

In practical terms, this means that a learner is reinforced for their behavior after a period of time that is not set but fluctuates. For instance, if a teacher praises a student after varying lengths of time following their participation in class, the student learns that they may receive reinforcement after some participation, but they cannot predict specifically when it will happen. This unpredictability can lead to a steady rate of response, as the learner is encouraged to keep engaging in the behavior in the hope of receiving reinforcement, regardless of when the reinforcement might occur.

In contrast, other schedules such as Fixed Ratio, Fixed Interval, and Variable Ratio involve different patterns of reinforcement that do not fit this description, either having predictable timings or being based on the number of responses made, rather than the passage of time alone.