Hawthorne effect

The Hawthorne effect is a form of reactivity. When the group which is under study knows about the study and their behavior changes after knowing that they are being studied, this change of behavior is the Hawthorne effect. For example: a group of 10 people are being studied for their health after exercising. Without knowing that they are being studied, they do or not do exercise, but after knowing that they are being studied they regularly do exercise.