Because the bar is always present in the Skinner box, the rat can respond to it as frequently or infrequently as it chooses. Hence rate of response is a useful measure of operant strength. The more frequently the response occurs during a given interval of time, the stronger it is.
The rate of response in operant conditioning is usually portrayed by a cumulative curve. Another measure of operant strength is the total of responses during extinction. A single reinforcement can produce considerable strength according to this measure.