194 CHAPTER 5
differ is in whether the number of responses or interval of time is fixed (the same in each case)
or variable (a different number or interval is required in each case). So it is possible to have
a fixed interval schedule, a variable interval schedule, a fixed ratio schedule, and a variable
ratio schedule (Skinner, 1961).
FIXED INTERVAL SCHEDULE OF REINFORCEMENT If you receive a paycheck once a week,
you are familiar with what is called a fixed interval schedule of reinforcement, in which
a reinforcer is received after a certain, fixed interval of time has passed. If Professor Conner
were teaching a rat to press a lever to get food pellets, she might require it to push the lever
at least once within a 2-minute time span to get a pellet. It wouldn’t matter how many times
the rat pushed the bar; the rat would only get a pellet at the end of the 2-minute interval if
it had pressed the bar at least once. It is the first correct response that gets reinforced at the
end of the interval.
As shown in Figure 5. 8 , a fixed interval schedule of reinforcement does not produce a
fast rate of responding (notice that the line doesn’t go “up” as fast as in the blue fixed ratio
line). Since it only matters that at least one response is made during the specific interval of
time, speed is not that important. Eventually, the rat will start pushing the lever only as the
interval of time nears its end, causing the scalloping effect you see in the graph. The response
rate goes up just before the reinforcer and then drops off immediately after, until it is almost
time for the next food pellet. This is similar to the way in which factory workers speed up
production just before payday and slow down just after payday (Critchfield et al., 2003).fixed interval schedule of
reinforcement
schedule of reinforcement in which
the interval of time that must pass
before reinforcement becomes possi-
ble is always the same.
Figure 5.8 Schedules of Reinforcement
These four graphs show the typical pattern of responding for both fixed and variable interval and ratio
schedules of reinforcement. The responses are cumulative, which means new responses are added to
those that come before, and all graphs begin after the learned pattern is well established. Slash marks
mean that a reinforcement has been given. In both the fixed interval and fixed ratio graphs, there is a
pause after each reinforcement as the learner briefly “rests.” The “scalloped” shape of the fixed interval
curve is a typical indicator of this pause, as is the stair-step shape of the fixed ratio curve. In the vari-
able interval and ratio schedules, no such pause occurs, because the reinforcements are unpredictable.
Notice that both fixed and variable interval schedules are slower (less steep) than the two ratio schedules
because of the need to respond as quickly as possible in the ratio schedules.