Which reinforcement schedule involves reinforcement occurring after an unpredictable number of responses?

The reinforcement schedule that involves reinforcement occurring after an unpredictable number of responses is the variable ratio schedule. This schedule is characterized by a pattern where reinforcements are provided after a varying number of responses, making it unpredictable for the individual when the next reinforcement will occur. This unpredictability leads to high rates of response, as the individual continues to respond in hopes of earning the next reinforcement.

For example, in a variable ratio schedule, a person might receive a reward after 5 responses one time, after 10 responses the next time, and after 3 responses another time. This variance keeps the individual engaged and motivated to continue responding, as they cannot predict exactly when the next reinforcement will be given, but they know it will eventually come after a certain number of responses.

This is in contrast to the other reinforcement schedules, where the timing or number required for reinforcement is predictable and consistent, such as fixed ratio, fixed interval, and variable interval schedules.