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To enhance the internal validity of a posttest only nonequivalent groups design, researchers can intentionally select comparison groups that share key characteristics. For instance, when evaluating a teaching method across two classrooms, a researcher might select classes within the same school that have similar baseline test scores and teachers with comparable styles to eliminate major confounds. However, because true random assignment is absent, unmeasured confounding variables may still influence the outcomes, meaning researchers cannot make definitive causal claims.

Controlling Confounds in a Posttest Only Nonequivalent Groups Design

In a posttest only nonequivalent groups design, why are researchers unable to make definitive causal claims even if they carefully select comparison groups that share key characteristics?

In a posttest only nonequivalent groups design, selecting comparison groups that share key characteristics guarantees the elimination of all confounding variables.

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