Many researchers are aware of the effect people have in certain scientific experiments. Their ideas and beliefs in particular can make the difference in whether a drug works for them, for example. The ideas, opinions, and beliefs of certain participants are known as producing experimenter bias, sampling bias, and subject bias. Although psychological research does their best to obtain data objectively, there’s no question that the beliefs of those participating in research play a role in whether data is accurate.
It’s the whole notion of the placebo effect. A placebo is a pill, medicine, or procedure that is harmless and has no therapeutic effect whatsoever. Instead, a placebo might be used to measure whether one’s belief in the medication has an effect on his or her health improving.