Observer expectancy bias happens when a researcher’s expectations, opinions, or prejudices influence what they perceive or record in a study. It often affects studies where observers are aware of the research aims and hypotheses.
Robert Rosenthal pioneered the study of this bias, which was first identified in collaboration with Kermit L. Fode when they wrote “The effect of experimenter bias on the performance of the albino rat” in 1963.
Such observer bias effects are near-universal in human data interpretation that are supposed to be objective, yet have expectations from the researcher. This causes the researcher to record data that conforms to their expectation, particularly in observational studies. Additionally, 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.
Double-blind techniques may be employed to combat the bias by causing the experimenter and subject to be ignorant of expectations.
Interpreting observational studies, the researcher usually taints the reported findings.
Clever Hans