A variable-ratio schedule delivers reinforcement after an unpredictable number of responses. The ratio requirement changes from one reinforcer to the next around an average value.

Slot machines and many forms of gambling operate on variable-ratio schedules: the next payoff could come after one more pull or after many. The schedule produces high rates of responding and extreme resistance to extinction. Because reinforcement is contingent on the number of responses rather than on the passage of time, faster responding yields faster reinforcement, encouraging persistent high activity.

In the laboratory, variable-ratio schedules generate steep, steady response rates with little pausing. The unpredictability of the requirement makes the absence of reinforcement after any given response uninformative, so extinction is slow. The same principle maintains a wide range of human behaviors that are only intermittently successful.

When the number of responses needed for the next reward keeps changing, responding tends to be rapid and persistent.