In operant conditioning, a simple reinforcement schedule in which reward follows the first response that the organism makes after a random time interval, and then the first response that it makes after another random time interval, and so on, the average length of the interval being specified in seconds as an affix to the abbreviation, hence VI20 indicates a variable-interval schedule with an average interval of 20 seconds. Also called a variable-interval reinforcement schedule. See also simple reinforcement schedule. Compare fixed-interval schedule, fixed-ratio schedule, variable-ratio schedule. VI abbrev.