In what way a posttest-only design with nonequivalent groups is better than a posttest-only design with no comparison group?

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In a posttest-only design with nonequivalent groups, participants in the experimental group are exposed to a treatment, the nonequivalent/control group is not exposed to the treatment, and then the two groups are compared. We must ensure the two groups are as similar as possible when using this design. As long as the similarity of the two groups is rigorously justified, the posttest-only design with nonequivalent groups is informative and powerful in estimating the treatment effect.