Experimental design
Experimental design uses control groups and treatment groups to study the effect of some intervention. It is used to minimize or eliminate confounding variables and allows us to understand the relationship between independent variables and the dependent variable.
For example, a psychologist studying the effects of violent video games on attitudes might divide a group of subjects into two, have one group play a violent video game (the treatment group), and have the other group play a non-violent video game (the control group). Then a questionnaire would be given to everyone to see if the treatment group were different from the control group as a result of playing the violent video game.