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A fixed interval schedule delivers reinforcement for the first target response after a consistent amount of time has passed since the prior reinforcer. In ABA, the rule is same time, then first response: time makes reinforcement available, while the specified behavior is still required.
These fictional examples are for exam study. In practice, an RBT follows the supervising BCBA's written plan and asks the supervisor about unclear procedures.
Fixed Interval Schedule Features and Implementation#
The RBT Test Content Outline (3rd ed.) states that task C.1 involves implementing positive and negative reinforcement procedures, including according to schedules of reinforcement. A fixed interval procedure may be part of a written plan when the plan identifies when a target response can contact reinforcement.
An RBT implementing a written fixed interval procedure needs to identify four parts:
- Target response: The behavior that must occur.
- Interval duration: The consistent amount of time that must pass.
- Qualifying response: The first target response after the interval ends.
- Programmed consequence: The reinforcer delivered after that qualifying response.
The schedule name does not replace the rest of the plan. The written procedure may also specify prompts, materials, data collection, the reinforcer, and conditions for delivery. For reinforcement implementation vocabulary, visit reinforcement procedures to review contingent consequence delivery.
A consistent amount of time#
A fixed interval schedule uses one stated duration for each opportunity. For example, FI 4 minutes means that a 4-minute interval begins after reinforcement is delivered. The interval does not vary from one opportunity to the next.
The first response after the interval#
The learner does not need to respond at the exact instant the timer reaches the stated duration. Rather, the first specified response after the duration has passed earns reinforcement.
Suppose a 3-minute interval ends at 10:03. If the learner emits the target response at 10:03:18, that is the qualifying response. If no target response occurs at 10:03, the reinforcer waits until the next target response occurs.
Reinforcement remains contingent on behavior#
A timer does not independently produce the reinforcer in an FI arrangement. If a token is delivered every 3 minutes whether or not the learner responds, the arrangement is fixed time, not fixed interval reinforcement. FI includes both a time requirement and a response requirement.
After the qualifying response receives the programmed consequence, the next interval begins as the procedure specifies. This cycle is why a response during the interval does not usually earn the scheduled reinforcer.
For a broader terminology reference, go to ABA terms to place fixed interval schedules among related concepts. For a wider overview, read schedules of reinforcement to compare time-based and response-based contingencies.
Fixed Interval Schedule Examples and Non-Examples#
| Situation | FI schedule? | Reason |
|---|---|---|
| The first independent greeting after every 5-minute interval earns praise. | Yes | The duration stays constant, and a response is required after it ends. |
| A token is delivered every 5 minutes regardless of behavior. | No | Time alone produces the token. |
| Every fifth independent greeting earns praise. | No | The requirement is a response count. |
| The first greeting after intervals of 2, 4, and 6 minutes earns praise. | No | Time is relevant, but the interval changes. |
| Every independent greeting earns praise. | No | No waiting interval is required. |
A quick way to classify an arrangement is to ask: Would the consequence occur if no target response happened? If yes, it is not FI. If a response is required, determine whether the time period remains fixed or changes across opportunities.
Fixed interval differs from fixed ratio because FI uses elapsed time, while fixed ratio uses a response count. A fixed ratio plan might provide reinforcement after every fourth response; an FI plan might provide reinforcement for the first response after every 4 minutes. Visit fixed interval vs fixed ratio for a direct comparison of these two schedules.
Variable interval also uses time, but its intervals change. An arrangement with varying waits of 2, 4, and 6 minutes is variable rather than fixed. Go to variable interval schedule to distinguish changing time periods from consistent ones.
Worked Timeline and the Scallop Pattern#
A teaching plan states: “Provide 1 minute of music after the first independent sorting response following each 3-minute interval.” At 10:00, Nia sorts an item independently and receives music.
- At 10:01, Nia sorts another item. This response occurs during the interval and does not earn the programmed music.
- At 10:02:40, Nia sorts again. The interval has not yet ended, so this response also does not earn music.
- At 10:03, the interval ends. Music is not delivered because a post-interval target response has not occurred.
- At 10:03:12, Nia independently sorts an item. This is the first target response after the interval, so the programmed music is delivered.
The rule is not “deliver music at 10:03.” The rule is “deliver music for the first specified response after 10:03.” This distinction separates fixed interval from fixed time.
In fixed interval psychology, a scallop is a possible response pattern in which responding is low or paused soon after reinforcement and rises as the next interval approaches. On a cumulative record, that pattern can appear as a curved rise.
A scallop is not part of the definition of a fixed interval schedule. The defining contingency is a consistent interval followed by reinforcement for the first target response afterward. Response patterns can differ depending on the learner, task, reinforcer, and teaching arrangement.
Practice Questions#
1. A written plan provides 2 minutes of drawing after the first independent hand raise following each 4-minute interval. The interval ends, and Leo raises his hand independently. What should the RBT do next?
A Correct. Leo made the first target response after the stated interval, so the programmed drawing time follows.
B The stem identifies Leo's hand raise as the first qualifying response, so another response is not required.
C The consequence follows the qualifying response, not the next timer endpoint.
D Withholding drawing would not follow the written contingency after the qualifying response.
2. Mira's plan provides a brief break for the first completed worksheet item after each 3-minute interval. An item was completed at 2 minutes 30 seconds, then another was completed at 3 minutes 10 seconds. What should the RBT do next?
A The first item was completed before the 3-minute interval ended.
B Correct. The item at 3 minutes 10 seconds is the first completed item after the interval.
C The stem describes one break for the first completed item after each interval.
D A five-item requirement would add a response-count contingency not stated in the plan.
3. A plan specifies access to a preferred activity after the first independent request following every 5-minute interval. Three minutes have passed since the last delivery, and Sam makes an independent request. What should the RBT do next?
A The specified 5-minute interval has not elapsed.
B The plan states a 5-minute interval, and the stem gives no reason to change it.
C Correct. Sam's request occurred during the interval, before the time requirement was met.
D Five requests would add a response-count requirement absent from the plan.
4. An FI 2-minute procedure provides a token for the first target response after each interval. The 2 minutes have elapsed, but no target response has occurred. What should the RBT do next?
A The stem requires a target response after the interval, so time alone does not earn a token.
B Restarting the timer would skip the opportunity for the first post-interval response to contact reinforcement.
C A response before the interval ended does not meet the stated requirement.
D Correct. The token follows the first target response after the 2-minute interval.
Try the free RBT practice test for additional original study scenarios.
FAQ
What is a fixed interval schedule in simple terms?
A fixed interval schedule reinforces the first target response after the same amount of time has passed. Responses occurring before the interval ends do not meet that schedule's requirement.
What is an example of a fixed interval schedule?
A plan may provide a token for the first independent response after every 4-minute interval. Once that token is delivered, the next 4-minute interval begins.
Is fixed interval the same as fixed time?
No. Fixed interval requires a target response after the time period ends. Fixed time delivers an event after a stated duration regardless of responding.
How does fixed interval differ from fixed ratio?
Fixed interval uses elapsed time, whereas fixed ratio uses a response count. FI means same time followed by the first response; fixed ratio means a stated number of responses.
What does a scallop mean in fixed interval psychology?
A scallop describes a possible pattern in which responding increases as the next fixed interval approaches. It is a response pattern rather than the definition of the schedule.