Performance in multiple fixed-interval schedules.

Duncan et al. (1972) worked with six pigeons in a small chamber.

Each bird faced two fixed-interval (FI) schedules in one session.

Pecks on the left key paid grain after 60 s. Pecks on the right key paid after 120 s.

The team switched the two parts every few minutes and counted how fast the birds pecked.

Birds pecked twice as fast on the 60-s key as on the 120-s key.

The length of each part (2, 4, or 8 min) did not change the speeds.

Response rates tracked how often grain came, not how long the part lasted.

Timberlake et al. (1987) later ran the same FI setup with adult humans.

Money replaced grain and a green light told them when the FI was running.

People also showed the classic FI “break-and-run” pattern, proving the rule works across species.

Tracey et al. (1974) used handwriting instead of key pecks.

Two adults wrote faster when money came every 30 s than every 60 s.

Again, rate followed the pay schedule, not the session length, matching the pigeon data.

When you set multiple FI schedules, clients will work harder where pay-offs come sooner, no matter how long each activity block lasts.

Use this to balance work and rest periods in skill-building or DRL programs.

Keep the richer schedule short and frequent to keep engagement high.