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Fixed interval vs fixed ratio is a time-versus-response-count distinction: fixed interval makes reinforcement available for the first target response after the same time period, while fixed ratio delivers reinforcement after the same number of target responses. The BACB RBT Test Content Outline (3rd ed.) places schedules under C.1.
Quick Comparison#
This table shows the one decision that separates fixed interval from fixed ratio before any example details matter.
| Criterion | Fixed interval | Fixed ratio |
|---|---|---|
| Definition | Reinforcement becomes available after the same time period, then follows the first target response. | Reinforcement follows the same number of target responses each time. |
| What controls eligibility | Time passing, plus a target response after that time. | A count of target responses. |
| What is reinforced | The first eligible response after the interval. | The response that completes the required count. |
| Common study error | Delivering reinforcement when the timer ends without a response. | Counting time because responses happened during a period. |
| Best stem clue | "First response after 2 minutes" or another repeated time period. | "Every fourth response" or another repeated response count. |
Start with schedules of reinforcement when a stem could be fixed, variable, ratio, or interval. The broader aba terms hub is useful when a schedule example also uses vocabulary such as extinction, motivating operations, or generalization.
Same Data Worked Two Ways#
The table below uses one response log and asks how the result changes if the written rule is FI 2 minutes or FR 3 responses.
| Opportunity | Target responses during the work period | If the rule is FI 2 minutes | If the rule is FR 3 |
|---|---|---|---|
| 1 | 0:40, 1:30, 2:10 | Reinforce 2:10, the first response after 2:00. | Reinforce 2:10, the third response. |
| 2 | 0:20, 0:50, 1:20, 1:50 | No FI delivery yet; no response occurred after 2:00. | Reinforce 1:20, the third response. |
| 3 | 2:05 only | Reinforce 2:05, the first response after 2:00. | No FR delivery yet; only one response occurred. |
| 4 | 0:25, 0:50, 1:10, 2:15 | Reinforce 2:15, the first response after 2:00. | Reinforce 1:10, the third response. |
The same behavior stream can produce different delivery points because the schedule asks a different eligibility question. Fixed interval asks whether the set time has passed before the response. Fixed ratio asks whether the set response count has been completed.
The C.1 reinforcement task page connects this distinction to the full task wording: positive and negative reinforcement procedures must be implemented immediately, contingently, and according to the written schedule.
Decision Rule For C.1 Study Scenarios#
This decision table turns the comparison into a fast check for original C.1 practice scenarios.
| If the scenario says... | Choose... | Reason |
|---|---|---|
| Same response count each time | Fixed ratio | The schedule is controlled by a fixed number of responses. |
| Same amount of time before eligibility | Fixed interval | The schedule is controlled by a fixed time period, then a response. |
| Changing response counts around an average | Variable ratio | The requirement is still count based, but it changes. |
| Changing time periods around an average | Variable interval | The requirement is still time based, but it changes. |
| A consequence appears by time regardless of behavior | Not FI or FR | A reinforcement schedule for C.1 keeps the consequence contingent on a target response. |
For the nearest variable comparison, use variable ratio vs variable interval after this page. That page separates changing counts from changing times, while this page keeps the focus on fixed requirements.
Use fixed interval schedule when the time side needs more examples. Use fixed ratio schedule when the response-count side is the confusing part.
Practice Allocation#
This table separates the official scored-exam blueprint from this article's small practice drill.
| Practice domain | Items on this page | Task tags used | Official scored-exam reference |
|---|---|---|---|
| C. Behavior Acquisition | 4 | C.1 | 19 scored questions, 25% |
These original questions practise implementation of a written schedule under C.1. They do not reproduce official exam wording or predict which scenarios will appear on an exam form.
Practice Questions#
The RBT Test Content Outline (3rd ed.) states C.1 as: "Implement positive and negative reinforcement procedures (e.g., immediately, contingently, according to schedules of reinforcement) along a continuum of dimensions (e.g., magnitude, intensity, variety)."
1. During matching, an RBT follows a teaching sheet that says to deliver a token after every fourth independent match. Mina completes four matches in a row. Which implementation matches the sheet?
A Correct. The written rule uses a fixed response count, and the fourth independent match completes that count.
B Waiting for a timer would add an interval requirement that is not in the teaching sheet.
C Changing to mixed counts would alter the stated schedule instead of implementing it.
D Delaying until another session would not deliver the scheduled consequence for the current response count.
2. During writing, help is available for the first independent sentence after each 3-minute timer. Kai writes one sentence at 2:10 and another at 3:20. Which response contacts help?
A The earlier sentence happened before the 3-minute interval elapsed, so it is not eligible under this rule.
B Correct. The 3:20 response is the first target response after the fixed time period elapsed.
C Both responses would contact reinforcement only if the rule called for each response to contact help.
D A response did occur after the interval elapsed, so the written rule has an eligible response.
3. A program says praise follows every fifth correct label. The learner labels five cards in 70 seconds. Which label should contact praise under the written rule?
A The first label starts the count but does not complete the stated response requirement.
B A timed label would matter for an interval rule, but this rule counts correct labels.
C Correct. The fifth correct label completes the fixed response count in the written rule.
D Waiting for a later label would exceed the stated count and change implementation.
4. In sorting, a token is available for the first correct sort after each 2-minute period. Sam makes correct sorts at 30, 70, and 130 seconds. What should the RBT do at 130 seconds?
A Counting three sorts would apply a ratio rule, but the written rule uses elapsed time plus a target response.
B Waiting for five responses would add a count requirement that is not in the schedule.
C Restarting the session would not implement the stated delivery rule for the eligible response.
D Correct. The 130-second sort is the first target response after the 2-minute period elapsed.
FAQ
What is the main difference between fixed interval and fixed ratio?
Fixed interval is based on time, then a target response. Fixed ratio is based on a set number of target responses.
What is a fixed ratio schedule?
A fixed ratio schedule delivers reinforcement after the same number of target responses each time, such as after every fourth independent response.
Can a fixed interval schedule deliver reinforcement when the timer ends?
No. In a fixed interval schedule, the elapsed time makes reinforcement available; the first target response after that time contacts reinforcement.
How is fixed interval vs fixed ratio different from variable schedules?
Fixed schedules keep the requirement the same. Variable schedules change the response count or time period around an average. The related stimulus vs response generalization distinction uses different vocabulary, but it is another example of separating similar ABA terms by the exact feature that changes.
Does an RBT choose the schedule during real services?
For study purposes, an RBT identifies and implements the schedule described in the scenario. During real services, schedule selection and changes follow the supervising BCBA's written plan. After this comparison, the rbt practice test behavior acquisition page mixes schedule questions with prompts, chaining, shaping, maintenance, generalization, and token-economy items from the rest of Domain C.
Sources
- RBT Test Content Outline (3rd ed.), updated 09/2024, overview and C.1 task statement.