What does "variable" indicate when discussing schedules of reinforcement?

When discussing schedules of reinforcement, "variable" refers to an unpredictable element wherein the reinforcement occurs after an average number of responses or time has elapsed. This means that while the reinforcement is delivered at different rates or intervals, the overall average can remain relatively consistent, resulting in a behavior pattern that is highly resistant to extinction.

In contrast to fixed schedules, which provide reinforcement after a set number of responses or at regular time intervals, variable schedules contribute to greater variability in the timing and frequency of reinforcement. This unpredictability tends to generate more consistent and sustained behaviors because the subject learns that reinforcement is available but does not know exactly when it will be delivered. This is why the response pattern under variable schedules is described as approximately average—responses fall within a range around this average, reinforcing the behavior in a less predictable but often more effective manner.