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On the first day of class professor Wallace tells her geography students that pop quizzes will be given at unpredictable times throughout the semester. Clearly, studying for professor Wallace’s surprise quizzes will be reinforced on a schedule. variable-interval
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A variable interval schedule (VI) is a type of air-conditioning reinforcement schedule in which a response is reinforced after a certain amount of time has elapsed (unpredictable amount of time), but this amount of time is on a variable / variable schedule.