In operant conditioning, a simple reinforcement schedule in which reward follows the first response that the organism makes after a predetermined time interval, and then the first response that it makes after the same interval, and so on, the duration of the interval being specified in seconds as an affix to the abbreviation, hence FI20 indicates a fixed-interval schedule with a 20-second interval. Also called a fixed-interval reinforcement schedule. See also simple reinforcement schedule. Compare fixed-ratio schedule, variable-interval schedule, variable-ratio schedule. FI abbrev.