What is the schedule of reinforcement where the reinforcer is delivered for the first response after a time period that varies around some average value?

The schedule of reinforcement described involves the delivery of a reinforcer after the first response that occurs following a time period that is not fixed but instead varies around an average value. This is characteristic of the variable interval schedule. In this type of schedule, the timing of the reinforcement is unpredictable, which means that the individual will engage in the behavior at a steady rate, anticipating that they will receive reinforcement at some point after a certain duration.

This variability in the time required for reinforcement leads to a more consistent response rate compared to fixed schedules, where reinforcement occurs at regular intervals. The unpredictability can sustain behavior more effectively, as the individual is motivated to respond, not knowing exactly when the reinforcement will arrive but knowing it will come at some point.

In contrast, fixed interval schedules have a set time before reinforcement is provided, variable ratio schedules involve reinforcement after an unpredictable number of responses, and fixed ratio schedules provide reinforcement after a fixed number of responses. Each of these schedules creates different patterns of behavior, but only the variable interval schedule directly aligns with the description provided in the question.