In listening tests, opinions that are formed about sound quality and stereo imaging are influenced by many factors in addition to the one that may be specific interest. Some of these are purely technical, others are psychological, and still others are related to the procedure employed in performing the tests. Some of all of them can cause opinions to be variable, changing from time to time, or inappropriate, influenced more by the -nuisance variables- than by the device under test. It is essential to be aware of these possibilities, especially if the results of the tests are to have meaning and validity outside of the specific circumstances under which they were conducted.
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