Observer bias

The observer effect refers to several things in different situations, though there are similarities.

Use in science
In science, the observer effect refers to changes that the act of observing has on the phenomenon being observed. For example: trying to observe an electron will change the path of the electron.

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 &mdash; until that time, the cat is both alive and dead.

In physics, a more mundane observer effect can be the result of instruments that by necessity alter the state of what they measure in some manner. For instance, in electronics, ammeters and voltmeters usually need to be connected to the circuit, and so by their very presence affect the current or the voltage they are measuring. Likewise, a standard mercury-in-glass thermometer must absorb some thermal energy to record a temperature, and therefore changes the temperature of the body which it is measuring.

The Heisenberg uncertainty principle is frequently, but incorrectly, confused with the "observer effect", as it relates precision in measurements related to to changes in velocity and position of certain particles relative to the perspective the observer takes on them.

Use in the social sciences
In the social sciences and general usage, the effect refers to how people change their behavior when it is observed and set down. People often do not behave in their usual manner when aware of being watched (see Hawthorne effect).

In the armed forces, an announced inspection is used to see how well soldiers can do when the put their minds to it, while a surprise inspection is used to see how well prepared they generally are.

In parapsychology, the observer effect refers to the phenomenon that when the person performing the tests (the phrase was coined by two friends performing an experiment wherein they set up a number of volunteers who had to press the button when they felt they were being watched by the experimenters) expects to get positive results, he does, and likewise when he expects negatives.

Observer bias
The related social-science term observer bias is error introduced into measurement when observers overemphasize behavior they expect to find and fail to notice behavior they do not expect. This is why medical trials are normally double-blind rather than single-blind. Observer bias can also be introduced because researchers see a behavior and interpret it according to what it means to them, whereas it may mean something else to the person showing the behavior. See subject-expectancy effect and observer-expectancy effect.