One of the major discoveries of operant conditioning was that not only do reinforcers have the power to cause responses to be made more often, but that how and when those reinforcers are delivered also affects the pattern of responses. Controlling the how and when of reinforcement is a reinforcement schedule.
Schedules are of two main types, time-based and response-based. Time-based schedules usually contain the word interval, as in time interval. Response-based schedules usually contain the word ratio, referring to the ratio of responses over time.
Fixed interval (FI) schedules reinforce any response made during an unchanging interval. For example, an FI-5 schedule would deliver a reinforcer every five seconds if at least one response had been made during the five seconds. Note that the FI-5 schedule would deliver just one reinforcer if many responses had been made during the five seconds.
Variable interval (VI) schedules are similar to FI schedules, except that the interval varies randomly for each reinforcer. So, a VI-5 schedule would deliver a reinforcer over an average interval of 5 seconds, not every five seconds. Each interval would be different.
Fixed ratio (FR) schedules deliver a reinforcer based upon a constant number of responses. For example, a FR-15 schedule would deliver a reinforcer every 15th response.
Variable ratio (VR) schedules are similar to fixed ratio, except that the number of responses required for a reinforcer changes each time. So, a VR-15 schedule would deliver a reinforcer over an average of 15 responses, not on every 15th response.