Fixed-interval schedule

Time opens the opportunity

On an FI 5-minute schedule, five minutes must pass before a qualifying response can produce the scheduled consequence. If the person responds at minute four, the interval continues. The first qualifying response after minute five produces reinforcement and usually starts the next interval.

The schedule definition needs five parts: the response, interval length, clock start, eligibility rule, and reset event. Without them, “every five minutes” can describe several different arrangements.

Fixed interval differs from fixed time

Under fixed time, an event occurs after a set period without a response requirement. A timer that announces a break every 20 minutes is fixed time. A system that makes a chosen feedback check available for the first completed step after each 20-minute interval is fixed interval.

This distinction matters for teaching and records. Calling fixed-time delivery “FI” implies a response-consequence contingency that is absent.

It also differs from a ratio schedule

Ratio schedules depend on a response count. Interval schedules depend on elapsed time plus an eligible response after the interval. Completing five forms can meet an FR 5 requirement. Completing the first form after five minutes can meet an FI 5-minute requirement.

Baum’s recent molar account reviews ratio and interval schedule definitions and common performance differences. It is a conceptual and laboratory-focused analysis, not a clinical prescription.

Response patterns are observations, not definitions

Textbooks often describe low responding after reinforcement followed by accelerating responding near the end of an FI interval. This “scallop” is a common laboratory pattern. It is not required for the schedule to be FI and may not appear with short intervals, added cues, human rules, competing activities, or different histories.

Measure the actual pattern instead of assigning one from the schedule label. Hackenberg’s token-reinforcement review describes schedule-typical responding in laboratory token systems while showing how exchange arrangements add complexity.

A fictional FI record

Priya chooses to receive a brief inventory-status check after completing work. The timer begins after each check. The first accurately completed inventory entry after five minutes produces the next check and resets the timer.

Across four intervals, a qualifying first response occurs after each interval and produces the scheduled check: 4 of 4 intervals. Priya also completes entries within the intervals, but those do not produce the scheduled consequence. The record shows implementation of the defined schedule. It does not show that FI caused any change in accuracy, speed, preference, or wellbeing.

Integrity requires a clock and event log

Record:

• the exact clock start and end for every interval
• each response and whether it met quality criteria
• the first eligible post-interval response
• consequence delivery and delay
• resets, pauses, interruptions, prompts, and invalid intervals
• the person’s preference and any withdrawal or discomfort

Schedule-integrity rate can be correctly delivered consequences divided by due consequences. Keep it separate from learner performance and outcome measures.

Clinical use should solve a real problem

An FI arrangement may support periodic checking, scheduled work review, or another agreed routine. It should not create needless waiting, hold ordinary communication hostage, or replace natural feedback that keeps a task safe and understandable.

Choose the interval from assessment and practical constraints. Consider whether a timer, visual cue, or self-management tool improves predictability. If early responses indicate confusion, teach the rule or change the arrangement instead of treating them as misconduct.

Food, water, bathroom access, AAC, mobility, prescribed care, rest, and emergency help remain available according to need. A schedule label does not justify withholding them.

The BACB BCBA Test Content Outline, 6th edition includes basic and compound reinforcement schedules in examination content. It does not specify an interval, reinforcer, or person-specific plan.

Evaluate more than response rate

Track task quality, response distribution within intervals, latency after interval completion, consequence delivery, preference, burden, generalization, and unwanted effects. Compare conditions only when their clocks, opportunities, and measurement rules are equivalent.

If the goal is durable performance, test whether natural signals and consequences can replace the programmed schedule. Preserve useful supports when they remain part of ordinary access.

Name schedule changes before combining data

An FI 5-minute phase and an FI 10-minute phase are different conditions. Mark the effective trial or date, preserve both series, and avoid averaging them as one schedule. If a timer cue is added, record that change too. Clear versioning lets reviewers distinguish a schedule effect from a simultaneous change in prompts, task, consequence, or staffing.