A research technique whereby the researcher watches the subjects of his or her study, with their knowledge, but without taking an active part in the situation under scrutiny. This approach is sometimes criticized on the grounds that the very fact of their being observed may lead people to behave differently, thus invalidating the data obtained, as for example in the famous case of the so-called Hawthorne effect. To overcome this, researchers normally observe a number of similar situations, over a period of time. Although video-recorders can now be used in non-participant observation, this too may alter (indeed almost certainly will alter) the behaviour of the research subjects. See also participant observation.