Editing Observer-expectancy effect (section)

== Overview ==
The experimenter may introduce cognitive bias into a study in several {{nowrap|ways{{px2}}{{mdash}}{{px2}}}}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.<ref>{{cite book |first=R. |last=Rosenthal |title=Experimenter Effects in Behavioral Research |location=NY |publisher=Appleton-Century-Crofts |year=1966}}</ref>

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 [[Elementary arithmetic|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. Ruling out simple fraud, Pfungst determined that the horse could answer correctly even when von Osten did not ask the questions. However, the horse was unable to answer correctly when either it could not see the questioner, or if the questioner themselves was unaware of the correct answer: When von Osten knew the answers to the questions, Hans answered correctly 89% of the time. However, when von Osten did not know the answers, Hans guessed only 6% of questions correctly. Pfungst then proceeded to examine the behaviour of the questioner in detail, and showed that as the horse's taps approached the right answer, the questioner's [[Human position|posture]] and facial expression changed in ways that were consistent with an increase in tension, which was released when the horse made the final, correct tap. This provided a cue that the horse had learned to use as a [[Reinforcement learning|reinforced]] cue to stop tapping.{{Citation needed|date=October 2023}}

Experimenter-bias also influences human subjects. As an example, researchers compared performance of two groups given the same task (rating portrait pictures and estimating how successful each individual was on a scale of −10 to 10), but with different experimenter expectations.{{Citation needed|date=October 2023}} In one group, ("Group A"), experimenters were told to expect positive ratings while in another group, ("Group B"), experimenters were told to expect negative ratings. Data collected from Group A was a significant and substantially more optimistic appraisal than the data collected from Group B. The researchers suggested that experimenters gave subtle but clear cues with which the subjects [[Asch conformity experiments|complied]].<ref>Rosenthal R. ''Experimenter Effects in Behavioral Research''. New York, NY: Appleton-Century-Crofts, 1966. 464 p.</ref>