A classic study was conducted by Elton Mayo, at the Hawthorne Works in Illinois. It was an overt non-participant observation or field experiment, in which workers' productivity was observed against a number of variables. It was later concluded that the most interesting conclusion to be drawn from this research was not relating to the variables Mayo set to test, but to the impact of being observed in the first place. Because of this observation, the phenomenon of the subjects of a study behaving differently because they are being studied has become known as the Hawthorne Effect. One way to try and avoid the Hawthorne Effect is to use covert observation so that the subjects of the study are unaware that they are being studied. However, this comes with a number of ethical concerns.