Hawthorne effect

From EvaluationWiki

The effect on a subject of knowing that s/he is a subject in an experiment. Sometimes used to include the complementary phenomenon, the effect of knowing that one is in the control group, which may lead to great increases in effort to excel, or to resignation; this is also sometimes called the 'reverse Hawthorne effect' or given another name altogether. These two, sometimes powerful, effects are what makes it so important to run double-blind studies whenever possible, but they are not immune from these effects. (Note: the effect's name is taken from the Western Electric plant where it was supposed to have been observed in the course of a study of changes in work lighting; recent research suggests that this story has no foundation in fact. It is also disputed what the exact phenomenon is that carries this name; see the wikipedia.org discussion under this heading.)