The reseachers systematically varied prices in the test supermarkets. Prices in the control town were not changed. The dependent variable measured was weekly sales at the beginning and end of the experiment. Let's diagram each of these two designs. A group that is not exposed to a treatment is called a control group - the control group is a special type of treatment group.
Treatment groups contain test units, who are the subjects of the experiment.
O represents an observation or measurement of the dependent variable made on a treatment group. Typically, multiple dependent variables will be measured. X. R represents random assignment of test units or treatment groups to separate treatments. At first glance, the laboratory experiment seems a much stronger design than the field experiment. The use of randomization allows us to rule out competing explanations, and determine that the differenence, d, was in fact due to the treatment, X. This is called internal validity.