Group 1 is randomized, pretested, given treatment, and then posted tested. Group 2 is randomized, pretested, given no therapy, and then post tested.
Randomization creates comparability
Advantage of using a pretest is that it permits researchers to determine how much each group has gained, not just whether they are different at the end of the experiment
However, sometimes having a pretest causes a problem because it can sensitize participants to the experimental treatment and, in effect, become part of the treatment.