An experiment cannot take place without a control group .
Experiments usually have two groups - the control group and the experimental group.
The experimental group is where the researcher manipulates the experimental variables. The control group is where variables are for the most part held constant. At the end of the experiment, the two groups are looked at side by side. The control group provides a basis of comparison that allows the researcher to better assess the effects of the experimental manipulation. In our example, we couldn't determine if an increase in violent behavior in the children was really due to violent video games without a control group.