Definition
Fixed Interval (FI) is a schedule of reinforcement where reinforcement is delivered for the first correct response after a set, fixed amount of time has passed. In this schedule, the interval of time between opportunities for reinforcement remains consistent, but only the first response that occurs after the interval is reinforced. This can result in a pattern of behavior where responses become more frequent as the end of the interval approaches.
Example
A teacher implements a Fixed Interval (FI) schedule where a student is given a token for staying on task every 5 minutes. After each 5-minute interval, the student must be on task to earn the token. If the student is not on task when the 5 minutes are up, no token is given, and the timer resets for the next interval. Over time, the student may begin to monitor the passing time and increase their on-task behavior as the end of the interval approaches.