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The one-group posttest-only design is a design that involves measuring the research participants on the dependent variable after they have undergone an experimental treatment condition. This design contains threats to internal validity because it does not include an equated comparison group so it is impossible to determine if the treatment condition had any effect. The one-group pretest-posttest design is a design where the treatment condition is interjected between two assessments of the dependent variable. This design is faulty because the only comparison is with the groups’ pretreatment behavior. Also, there is no attempt to control for the effect of confounding extraneous variables because there is no equated comparison group. The nonequivalent posttest-only design is a design in which the performance of the experimental group is compared with a nonequivalent control group. The nonequivalent posttest-only design is faulty because the treatment and control groups were not randomly assigned which means that they could have differed on the dependent variable prior to administration of the treatment condition.
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