Observer-expectancy effect

The observer-expectancy effect is a form of reactivity in which a researcher's cognitive bias causes them to subconsciously influence the participants of an experiment. Confirmation bias can lead to the experimenter interpreting results incorrectly because of the tendency to look for information that affirms their hypothesis, and overlook information that conflicts with it. It is a significant threat to a study's internal validity, and is therefore typically controlled using a double-blind experimental design.

The observer-expectancy effect is distinct from related phenomena such as the subject-expectancy effect and demand characteristics. In observer-expectancy effects, the researcher’s expectations influence participant behavior or data interpretation through subtle cues, whereas subject-expectancy effects arise from participants’ own beliefs about the study, and demand characteristics refer more broadly to situational cues that signal expected responses.

It may include conscious or unconscious influences on subject behavior including creation of demand characteristics that influence subjects, and altered or selective recording of experimental results themselves.

Overview

The experimenter may introduce cognitive bias into a study in several ways‍—‍in the observer-expectancy effect, the experimenter may subtly communicate their expectations for the outcome of the study to the participants, causing them to alter their behavior to conform to those expectations. Such observer bias effects are near-universal in human data interpretation under expectation and in the presence of imperfect cultural and methodological norms that promote or enforce objectivity.

The classic example of experimenter bias is that of "Clever Hans", an Orlov Trotter horse claimed by his owner von Osten to be able to do arithmetic and other tasks. As a result of the large public interest in Clever Hans, philosopher and psychologist Carl Stumpf, along with his assistant Oskar Pfungst, investigated these claims.