ABA glossary
Variable interval
Also called VI
Definition
A schedule in which the first response after an amount of time that varies around an average is reinforced. On VI 2 min, reinforcement becomes available after unpredictable intervals that average 2 minutes.
In plain words
Because the learner cannot tell when reinforcement will become available, steady responding is the best way to catch it. VI schedules produce moderate, steady rates without the scallop of a fixed interval. Responding faster does not make reinforcement arrive sooner, so rates are lower than on ratio schedules. Checking your phone for a message that could arrive at any time is an everyday example.
Example from a session
Owen's plan calls for praise for working on a VI 2-minute schedule. The RBT uses an app that vibrates at varied times averaging 2 minutes (for example 1, 3, 2, and 2 minutes). After each vibration she praises Owen the next time she sees him working. Owen works steadily, because he cannot predict when the next praise will come.
For BCBA candidates and supervisors
VI schedules are practical for maintaining behavior with modest staff effort, since the person delivering reinforcement only needs to watch for the behavior after each varied interval. Concurrent VI schedules are also the classic arrangement behind the matching law: relative rates of responding tend to match relative rates of reinforcement across the options. A limited hold can be added to require a response within a short window once reinforcement becomes available.
Often confused with variable ratio, variable-time schedule
VI and VR both feel unpredictable, but VI depends on time passing and then one response, while VR depends on how many responses occur. On a variable-time schedule the stimulus arrives at varied times with no response required, so it is noncontingent and not VI.
Exam tip
Unpredictable timing plus a required response means VI, and the expected pattern is a moderate, steady rate. If no response is required, the answer is variable time.
Can you spot variable interval in a scenario?
Practice BCBA B.9 with exam-style questions and instant feedback.