ABA glossary
Fixed interval
Also called FI
Definition
A schedule in which the first response after a fixed amount of time has passed is reinforced. On FI 5 min, responses during the 5 minutes earn nothing; the first response after 5 minutes is reinforced.
In plain words
Time decides when reinforcement becomes available, but a response is still required: nothing is delivered just because the interval ended. Learners on FI schedules often respond little right after reinforcement and speed up as the end of the interval approaches. On a cumulative record this looks like a curve called a scallop. Checking the oven more often as the baking time runs out is a familiar example.
Example from a session
During independent work, a silent timer that only the RBT can see runs for 4 minutes. The first time Jayden asks "Can you check my work?" after the timer goes off, the RBT checks it, praises him, and restarts the timer. Requests before then do not produce a check. Over the week Jayden rarely asks right after a check and asks more often as the 4 minutes run out, a scallop-like pattern the BCBA can see in the data.
For BCBA candidates and supervisors
The classic FI scallop comes mostly from laboratory work; people often respond differently because clocks, rules, and instructions affect their behavior. A limited hold can be added so reinforcement stays available only for a short window after the interval ends, which tightens responding. In applied settings, analysts choose interval schedules when reinforcement is naturally tied to time periods and make sure staff can run the timing consistently.
Often confused with fixed-time schedule, fixed ratio
On a fixed interval, reinforcement still depends on a response after the time ends. On a fixed-time schedule, the stimulus is delivered when the time ends whether or not the person responds, so it is noncontingent. On a fixed ratio, time does not matter; only the count does.
Exam tip
"The first response after" is the signature of an interval schedule. If the stem says reinforcement comes "every 5 minutes regardless of behavior," it is fixed time, not FI. If it mentions a scalloped pattern, choose FI.
Can you spot fixed interval in a scenario?
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