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Variable ratio and variable interval schedules differ in what makes reinforcement available: variable ratio depends on a changing number of responses, while variable interval depends on changing amounts of elapsed time. Both are variable schedules, but one is response-based and the other is time-based.
Variable Ratio vs Variable Interval at a Glance#
| Criterion | Variable ratio (VR) | Variable interval (VI) |
|---|---|---|
| Definition | Reinforcement follows a varying number of target responses. | Reinforcement is delivered for the first target response after varying time periods. |
| What controls availability | Response count. | Elapsed time. |
| What changes | The number of responses required. | The duration of each interval. |
| Effect of faster responding | It can complete the current response requirement sooner. | It cannot make the interval end sooner. |
| Example | Requirements may be 2, then 4, then 1 responses. | Intervals may be 2, then 4, then 1 minutes. |
| Study clue | Look for a changing response number. | Look for changing time plus a later response. |
The key difference between variable ratio and variable interval is the unit that changes. Ask whether the plan changes a response count or a clock requirement.
The RBT Test Content Outline (3rd ed.) lists C.1 as implementing positive and negative reinforcement procedures according to schedules of reinforcement. A written plan from the supervising clinician determines how a schedule is implemented.
For a larger framework, visit ABA terms to place reinforcement schedules among other foundational concepts. Review schedules of reinforcement to compare ratio, interval, fixed, and variable arrangements. For C.1 implementation language, use reinforcement procedures to connect schedule labels with consequence delivery.
A Matched Example#
Suppose target responses occur at minutes 1, 3, 4, 8, 9, 10, 14, 15, 20, 21, 25, and 26. Use the same sequence, 2, 4, 1, and 5, to see how a VR schedule and a VI schedule differ.
Variable ratio: the sequence is a response requirement#
Under a VR schedule with requirements of 2, 4, 1, and 5 responses:
- The second target response earns reinforcement at minute 3.
- The next requirement is 4 responses. The fourth response after that delivery occurs at minute 10.
- The next requirement is 1 response. The response at minute 14 earns reinforcement.
- The next requirement is 5 responses. The fifth response occurs at minute 26.
Time passes during a VR schedule, but time does not complete the requirement. The current response count must be completed before reinforcement is delivered.
Variable interval: the sequence is a time requirement#
Under a VI schedule with intervals of 2, 4, 1, and 5 minutes:
- Reinforcement becomes available after 2 minutes. The first listed response after that point is at minute 3.
- The next interval ends at minute 7. The first later response occurs at minute 8.
- The next interval ends at minute 9. The response at minute 9 contacts reinforcement.
- The next interval ends at minute 14. The response at minute 14 contacts reinforcement.
Responses during a running VI interval do not shorten the interval. Once the time requirement has elapsed, the first target response produces reinforcement.
A Three-Question Decision Rule#
Use these questions when deciding between variable interval vs. variable ratio reinforcement:
- Is the changing requirement counted or timed? A changing number of responses indicates a ratio schedule. A changing period of time indicates an interval schedule.
- Does the value repeat or change? A repeating value is fixed. A changing value around an average is variable.
- Which response earns reinforcement? On VR, it is the response that completes the current count. On VI, it is the first response after the interval ends.
A phrase such as “after an average of four responses” suggests variable ratio. A phrase such as “after an average of four minutes” suggests variable interval.
For fixed versions of the same distinction, read fixed interval vs fixed ratio because it compares time-based and response-based requirements that stay constant. Use variable ratio schedule for a response-based definition and variable interval schedule for a time-based definition.
Practice Questions#
1. A written plan uses a variable ratio sequence of 2, 4, 1, and 5 target responses. After a token delivery, the next requirement is 4 responses. Eli has made 3 target responses. What should the RBT do after the third response?
A The stated requirement is four responses, so the third response does not complete it.
B Correct. The fourth target response completes the current requirement.
C The plan gives a response requirement, not a timed interval.
D Resetting the count would prevent completion of the stated requirement.
2. A written plan specifies that the next variable interval lasts 3 minutes. Maya makes target responses at minutes 1 and 2, then again at minute 4. What should the RBT do at minute 4?
A The plan uses elapsed time rather than a response count.
B Restarting before reinforcement would postpone praise even though the minute-4 response satisfies the current interval.
C Correct. The interval ended at minute 3, and the minute-4 response is the first later response.
D The time requirement has not elapsed when the timer starts.
3. A written plan uses a variable ratio sequence of 1, 3, 2, and 4 responses. Following a reinforcer delivery, the current requirement is 1 response. Zara produces one target response. What should the RBT do next?
A Correct. One response completes the current stated ratio requirement.
B The stem describes a response-count requirement, not a timed interval.
C Timing is not part of the stated schedule.
D Additional responses would change the current requirement of one response.
4. A written plan states that the next variable interval is 2 minutes. Devon makes a target response 1 minute after the last reinforcer delivery. What should the RBT do at that moment?
A The response occurs before the stated interval has elapsed.
B Counting responses would add a ratio requirement not described in the plan.
C The stem does not state that this response restarts the interval.
D Correct. Reinforcement is not yet available because the interval is still running.
For a separate distinction involving behavior across conditions, read stimulus vs response generalization to separate generalization concepts from reinforcement schedules. Try the free RBT practice test.
FAQ
What is the difference between variable ratio and variable interval?
Variable ratio schedules change the number of responses required for reinforcement. Variable interval schedules change the amount of time that must pass before reinforcement is available for the first subsequent target response.
Does faster responding produce reinforcement sooner on both schedules?
Faster responding can complete a current VR requirement sooner. On a VI schedule, faster responding cannot make the interval elapse faster, although a target response is needed after the interval ends.
How do fixed and variable interval schedules differ?
Both schedules are based on elapsed time. A fixed interval repeats the same time value, while a variable interval changes the time values around an average.
What does variable mean in a reinforcement schedule?
Variable means the requirement changes from one opportunity to the next around an intended average. The changing unit may be a response count or elapsed time.