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Observer Expectancy Effect. The observer expectancy effect, also called the experimenter expectancy effect, is a phenomenon that can occur when a researcherâs beliefs or expectations cause him or her to unconsciously influence the participants of an experiment. In order to conserve the internal validity of an experiment, researchers can use a double-blind experimental design.
In the design of experiments, the experimenter is often interested in the effect of some process or intervention (the treatment) on some objects (the  experimental units ), which may be people, parts of people, groups of people, plants, animals, etc.