Every field of experimentation has now, of necessity, developed special tricks and precautions to avoid artifacts and spurious conclusions. No living organism remains the same throughout the day, or even during a few minutes the experimentation. Apprehension and excitement at the beginning of the run may give way to boredom and fatigue at the end. 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. The comparison of Condition A with Condition B for learning lists of nonsense syllables (see Chapter 21) sometimes may best be made by following an ABBA order to balance out the factors of inexperience at the beginning and boredom at the end of the experiment. Similarly, sensory experiments may be designed in such a way that progressive changes in adaptation or alertness are also counterbalanced. This may often be done by randomizing the order of presentation of stimuli of different strengths.