The posttest only nonequivalent control group design is used to show that a treatment was effective in a preexisting treatment group. ... One group is measured after receiving the 'treatment', and another control group who did not receive the treatment is measured at the same time.

Why do researchers use the non equivalent design?

When participants are not randomly assigned to conditions, however, the resulting groups are likely to be dissimilar in some ways. For this reason, researchers consider them to be nonequivalent.

What is the advantage of the pretest in the nonequivalent control group?

What are the advantages and disadvantages of the nonequivalent control group pretest-posttest design? Advantages: Can compare scores before and after a treatment in a group that receives the treatment and in a group that does not. Disadvantages: susceptible to the threat of selection differences.