Rates and patterns of responding with concurrent fixed-interval and variable-interval reinforcement.

Pigeons pecked two keys at the same time. One key paid off on a fixed-interval schedule. The other paid on a variable-interval schedule.

The team watched how many pecks landed on each key and how the birds spaced those pecks through time.

The birds did not match their pecks exactly to the pay rate. Instead, the ratio of pecks followed a power curve with an exponent of 0.5.

On the fixed-interval key, the birds showed the usual scallop, but the shape of that scallop changed when the overall pay rate changed.

Rapport et al. (1996) later used the same two-key setup but swapped the regular VI for exponential VI schedules. They still saw under-matching, yet the sensitivity slope grew steeper, showing the 0.5 exponent is not fixed.

Thomas (1968) had already split fixed-interval performance into orderly parts. Williams (1971) moved that idea into a concurrent setup and showed those parts shift with relative pay.

Leslie (1981) zoomed in on single VI schedules and found local response rate tracks local reinforcement chance. Williams (1971) echoes this: when FI and VI run together, local FI patterns also bend to local pay odds.

When you set up two schedules at once, clients rarely match behavior to payoff in a one-to-one way. Expect a softer, power-curve fit and watch how timing micro-patterns drift as you raise or lower the overall reinforcement rate. Use this to fine-tune concurrent schedules in token boards, choice tasks, or DRO blends.