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The Subject-expectancy effect, in science, is a cognitive bias that occurs in science when a subject expects a given result and therefore unconsciously manipulates an experiment or reports the expected result. Because it can skew the results of experiments (especially on human subjects), double-blind methodology is used to eliminate the effect.
Like the Observer-expectancy effect, it is often a cause of "odd" results in many experiments. But the Subject-expectancy effect is most commonly found in medicine, where it is called the Placebo effect. The typical example is a patient given a sugar pill, who experiences a reduction in pain because they were told the sugar pill was a pain reliever.