Fixed interval
A schedule of reinforcement in which the first response after a set amount of time has passed is reinforced, for example the first response after 2 minutes (FI 2-min).
On a fixed-interval (FI) schedule, a timer starts after a reinforcer is delivered. Responses made before the interval ends produce nothing. The first response after the interval has ended is reinforced, and the timer starts again. An FI 5-minute schedule means the first correct response after 5 minutes earns reinforcement. Time alone does not produce the reinforcer: the behavior still has to occur once the interval is over.
FI schedules tend to produce a recognizable pattern called a scallop. Right after reinforcement there is a pause with little responding, and responding speeds up as the end of the interval gets closer. A student who knows the teacher checks work only at the end of every 10 minutes may work slowly at first and quickly near the check.
In applied settings, a supervisor may use interval schedules when it is practical to check behavior at certain times rather than after a count of responses. The RBT's job is to time the interval accurately, deliver the reinforcer only for the first target response after the interval ends, and record what happened. Delivering the reinforcer when the timer goes off, whether or not the behavior occurred, is a different procedure (a fixed-time schedule, used in noncontingent reinforcement).
Example and non-example
| Example | The plan calls for FI 3-min praise for hand raising. Once 3 minutes have passed, the RBT praises the client's next hand raise and restarts the timer; hand raises before the 3 minutes are up are not praised. |
|---|---|
| Non-example | The RBT praises the client every 3 minutes when the timer beeps, regardless of what the client is doing. That is time-based delivery, not a fixed-interval schedule. |
Don't confuse it with
| Fixed ratio | Fixed ratio requires a set number of responses; fixed interval requires a set amount of time to pass before one response is reinforced. |
|---|---|
| Noncontingent reinforcement (NCR) | Noncontingent reinforcement delivers reinforcers on a time schedule regardless of behavior, while FI still requires a response after the interval. |
Test yourself
-
Question 1The plan states: "Praise the first time the client asks a peer to play after 2 minutes have passed since the last praise. Requests made before 2 minutes are up are not praised." Which schedule is this?
Show answer
D: Fixed interval 2 minutes. On a fixed interval schedule, the first response after a set amount of time has passed is reinforced. Responses before the interval ends are not reinforced.
- A: A fixed ratio 2 would reinforce every second request, regardless of time.
- B: A fixed-time schedule delivers reinforcement every 2 minutes whether or not the client responds.
- C: A variable interval schedule uses intervals that change around an average, not a set 2 minutes.
Task C.1 · 40-hour curriculum section A
-
Question 2Which response pattern is typical of behavior on a fixed interval schedule?
Show answer
B: Little responding right after reinforcement, with responding speeding up as the end of the interval nears. Fixed interval schedules typically produce a pause after reinforcement and a gradually increasing rate as the end of the interval nears, often called a scallop on a cumulative record.
- A: High, steady responding with few pauses is typical of variable ratio schedules.
- C: A pause followed by a fast run until a set count is reached is typical of fixed ratio schedules.
- D: Moderate, steady responding is typical of variable interval schedules.
Task C.1 · 40-hour curriculum section A
-
Question 3What is the main difference between ratio and interval schedules of reinforcement?
Show answer
B: Ratio schedules depend on the number of responses; interval schedules depend on time passing.. On a ratio schedule, reinforcement is delivered after a number of responses. On an interval schedule, reinforcement is delivered for the first response after an amount of time has passed.
- A: Both kinds of schedules can be used at different stages of teaching; neither is limited to maintenance.
- C: Any type of reinforcer can be delivered on either kind of schedule.
- D: Both ratio and interval schedules can be fixed or variable.
Task C.1 · 40-hour curriculum section A
Answer every question to see your score.
On the RBT exam
This term belongs to task C.1 Positive and negative reinforcement in content area C, Behavior Acquisition, which is 19 of the 75 scored questions.