Which schedule of reinforcement is characterized by unpredictable rewards after an unspecified number of responses?

The schedule of reinforcement that features unpredictable rewards after an unspecified number of responses is known as variable ratio. In this schedule, the reinforcement is provided after a varying number of responses, making it impossible for the subject to predict when the reward will occur. This unpredictability often leads to high rates of responding, as the learner continues to engage in the behavior in hopes of obtaining the next reward.

For example, think of a slot machine in a casino that pays off after a random number of plays. Some players might win after a few pulls, while others might need to play many times before winning. This creates a strong and persistent behavior since individuals are motivated by the potential of a reward that could come at any time, reinforcing their ongoing participation.

In contrast, the other schedules of reinforcement have specific characteristics that do not involve this level of unpredictability, which is what clearly distinguishes variable ratio from fixed interval, fixed ratio, and continuous schedules.