In operant conditioning, a simple reinforcement schedule in which reward is delivered after the organism has made a random number of responses, and is then rewarded after a further random number of responses, and so on, the average number of responses before reinforcement being specified as an affix to the abbreviation, hence VR50 indicates a variable-ratio schedule with an average of 50 responses between reinforcements. Also called a variable-ratio reinforcement schedule. See also simple reinforcement schedule. Compare fixed-interval schedule, fixed-ratio schedule, variable-interval schedule. VR abbrev.