Observer-expectancy effect

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The observer-expectancy effect (also called the experimenter-expectancy effect, expectancy bias, observer effect, or experimenter effect) is a form of reactivity in which a researcher's cognitive bias causes them to unconsciously influence the participants of an experiment. It is a significant threat to a study's internal validity, and is therefore typically controlled using a double-blind experimental design.

An example of the observer-expectancy effect is demonstrated in music backmasking, in which hidden verbal messages are said to be audible when a recording is played backwards. Some people expect to hear hidden messages when reversing songs, and therefore hear the messages, but to others it sounds like nothing more than random sounds[citation needed]. Often when a song is played backwards, a listener will fail to notice the "hidden" lyrics until they are explicitly pointed out, after which they are obvious. Other prominent examples include facilitated communication and dowsing.

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