In science, observer effect is the term for how someone observing and measuring an effect can change the thing being observed.
In quantum mechanics, if the outcome of an event has not been observed, it exists in a state of superposition, which is being in all possible states at once. The most famous example is the thought experiment Schrödinger's cat, in which the cat is neither alive nor dead until observed — until that time, the cat is both alive and dead.
In the social sciences, and in general usage, the effect refers to how people change their behaviour when it is observed and set down. People often do not behave in their usual manner when aware of being watched.
Observer bias
The related social science term Observer bias is error introduced into measurement when observers overemphasize behaviour they expect to find and fail to notice behaviour they do not expect. This is why medical trials are normally double blinded rather than single blinded. Observer bias can also be introduced because researchers see a behaviour and interpret it according to what it means to them, whereas it may mean something else to the person showing the behaviour.
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