Variable Ratio (VR) is a schedule of reinforcement in which reinforcement is provided after an unpredictable number of responses, but the average number of responses required for reinforcement remains consistent over time. This schedule tends to produce high rates of behavior because the reinforcement is not tied to a fixed number of responses.
Example
In a VR 5 schedule, a student might be reinforced for raising their hand after 3 responses, then after 7 responses, and then after 5 responses, with the average number of responses being 5. The unpredictability of when the reinforcement will come encourages a high rate of responding.