VI schedules used intervals selected in irregular order from a set of intervals, often described by a mathematical progression (e.g., arithmetic, geometric). Current practise favours schedules with a constant reinforcement probability over time within the interval (with probability measured by reinforcers per opportunity, i.e., probability that a response will be reinforced at a given time in an interval, given that the organism has reached that time). Such conditions are met by a type of VI schedule called random interval (RI), which arranges a setup (makes the next response eligible to produce a reinforcer) with a fixed probability every t seconds. In RI schedules, the average interval equals t divided by the probability (e.g., arranging a setup once per second with a probability of 0.02 produces RI 50-s). In one version, the schedule stops operating after a setup until the scheduled reinforcer is produced, so that low response rates make the obtained reinforcement rate lower than what had been scheduled; in another, the schedule continues to operate and successive setups accumulate, so that obtained and scheduled reinforcement rates remain about equal even with low response rates.