Dark adaptation can only be tested by the use of light, and certain perceptual phenomena can only be reported if the subject learns and remembers the appropriate words with which to describe them. Similarly, an overly cooperative subject may report detecting an odor when none is present, while a less alert subject may overlook test stimuli even when they approach moderate strength. The basic remedy in all these situations is so to design the experimental procedure that the uncontrolled variations in performance have a minimal effect on the final conclusions drawn from the experiment. Counterbalancing the order of presentation is one of the most valuable features of experimental design.