Used in teaching and proper use of reinforcers, fixed interval is a way to describe how much time must pass before a learner’s first correct response will be reinforced.  For example, a FI-15 is a way to label a schedule in which the learner’s first correct response after 15 minutes will be reinforced. Behaviors on a FI schedule show a “scallop pattern.”  Imagine a term paper that is due every 3 months (i.e., FI-3). As a student, you are less likely to work on the paper on the first day of a new trimester since “there is no need for you to do it.” However, as the trimester drags on and as you get closer to the deadline, you find yourself working as hard as you can until you complete the paper right before the deadline.  Have you ever heard of people described as “procrastinators?”

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