Fixed-interval and fixed-ratio reinforcement schedules with human subjects.

Stoddard et al. (1988) asked adults to press buttons under two rules. Press fast on one button. Press slowly on the other. The slow button paid only if the press came inside a short window near the end of a fixed interval.

The team first let people count seconds out loud. Then they added a tiny window—just a few seconds—where a press would count. They wanted to see if the window would stop people from counting and start pressing more often.

When the short window appeared, counting stopped. People no longer waited until the last second. They pressed steadily through the whole interval.

The schedule, not the clock in their head, now controlled the presses. The brief limited-hold broke the self-timing trick and put behavior back under the timer.

Rapport et al. (1982) and Sanders (1969) ran similar FR-FI games but gave people all the time in the world. They saw the same fast-slow split, yet people still used silent counting. Stoddard et al. (1988) kept the same schedules and just clipped the response window—wiping out the count strategy.

Harzem et al. (1978) tried the opposite move: they paid people for long pauses. That also wrecked the pause, showing whichever contingency is tightest wins. Stoddard et al. (1988) confirms the rule from the other side—tightening the window beats self-timing.

Okouchi (2003) later showed that past schedules can color today’s FI rate. Stoddard et al. (1988) adds that a tiny tweak in the current schedule can override any history.

If your learner is ‘waiting out’ an interval prompt, don’t just talk about it—tighten the window. Add a brief limited-hold to the DRO, DRL, or FI. The learner must respond a little earlier each cycle. The clock-watching melts away and steady responding returns. One small timer change can save many session minutes.

On a fixed-interval (FI) schedule, reinforcement is delivered for the first response after a fixed amount of time has elapsed since the last reinforcer. On an FI 5-minute schedule, responses during the 5 minutes do nothing; the first response after the interval ends produces reinforcement. Two features are easy to miss: a response is still required (time alone delivers nothing), and only one response is required once the interval is up.

The signature performance in nonhuman research is the FI scallop: a pause after each reinforcer followed by gradually accelerating responding as the end of the interval approaches. The cumulative record curves like a scallop shell, which is why the pattern carries that name on BCBA exam materials.

Do not confuse FI with a fixed-time (FT) schedule. On FT, the reinforcer is delivered after the interval regardless of behavior, with no response requirement, which makes FT a response-independent schedule used in procedures like noncontingent reinforcement. If a response is required, it is FI; if the reinforcer shows up on the clock no matter what, it is FT.

On a fixed-ratio (FR) schedule, reinforcement follows every nth response: FR 10 means every tenth response pays. The classic FR pattern is break-and-run, a post-reinforcement pause followed by a high, steady run of responding to the next reinforcer. Piecework is the standard everyday analogy, since finishing ten units produces payment regardless of how long it took.

The contrast is what schedules control: FR contingencies count responses, so response rate determines how quickly reinforcement arrives, and rates run high. FI contingencies clock time, so responding faster earns nothing sooner, and efficient FI performance is actually minimal, just one well-timed response per interval.

In this 1988 report, a multiple schedule alternated FR and FI components with adult human subjects, ward staff on a psychiatric research unit, allowing direct observation of acquisition, maintenance, and transitions to larger schedule values in the same individuals.

The subjects in this study did something no pigeon can: they used their watches. Rather than scalloping, they timed the interval and typically emitted a single response right when it ended, collecting the reinforcer with almost no wasted effort. Human FI performance is notoriously unlike the animal pattern, often because self-generated rules and timing devices, or counting, mediate the schedule contact.

The experimenters' countermeasure was a limited hold: the reinforcer was only available for a short window after the interval elapsed, and a response outside that window missed it. Short limited-hold values eliminated the clock watching and increased FI response rates, pulling behavior back under the schedule's control. The subjects also talked to each other freely, and their performance reflected both the programmed contingencies and that verbal exchange.

For BCBAs, this is a compact lesson in why schedule effects from the animal lab transfer imperfectly to verbally able humans: rules, instructions, and timing strategies compete with direct contingency control. When a client's behavior does not match the textbook schedule pattern, look for the mediating behavior, because there usually is one.