There are four basic schedules of reinforcement based on these two distinctions: Fixed-ratio schedules involve reinforcement after a fixed number of responses after every tenth response, for example. In this case nine non-reinforced responses would be followed by one reinforced response. If you had a job in which you were paid for every twenty pieces of work that you turned out, you would be working on a fixed-ratio schedule. The machine pays after a certain number of responses—say, once every ten handle pulls, on the average but the gambler does not know in advance which particular attempt will pay off. This schedule of reinforcement has proved to be highly effective in keeping gamblers at the machine.

**With fixed-interval schedules,** the first response after a set period of time gets reinforced. The number of responses made during the interval does not matter. A rat on a fixed-interval schedule will increase the number of bar presses as the time for reinforcement draws near. Similarly, students are likely to respond to the fixed intervals at which examinations take place by cramming just before the exam, laying off right afterward, and starting to study again as the next exam approaches.

**With variable-interval schedules**, the only response reinforced is the first one after a time interval has passed, but the length of the interval varies around some average. An animal pressing a bar might be rewarded after intervals averaging out to five minutes, with some intervals as short as a minute and some as long as fifteen minutes. Similarly, an instructor might have you on a variable-interval schedule for studying by giving you surprise quizzes. The quizzes may come once a week on the average, but some come a day after the last one, and others come as long as two weeks after. A variable-interval schedule typically produces a steadier rate of responding than does a fixed-interval schedule, as when a student has to study regularly to be prepared for pop quizzes.

In real life it is difficult to find pure examples of these four basic schedules. The schedules of partial reinforcement that most often occur are mixed schedules that are combinations of ratio and interval, fixed and variable. You may be paid on the first and fifteenth days of every month (fixed interval), but pay raises are more likely to be determined by the number of responses—such as the number of encyclopedias you sell—which is more like a variable-ratio schedule.