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A nonequivalent groups design is a specific type of between-subjects design and a quasi-experimental approach in which researchers do not use random assignment to allocate participants to their respective conditions. Due to this absence of random assignment, the resulting comparison groups are nonequivalent, meaning they are likely to have inherent, pre-existing differences. These initial dissimilarities introduce potential confounding variables that can significantly threaten the internal validity of the research.

Nonequivalent Groups Design

A posttest only nonequivalent groups design is a quasi-experimental approach where one group receives a treatment and is then compared to a nonequivalent control group that did not receive the treatment. Because the researcher does not use random assignment to allocate participants to these groups, inherent differences may exist between them from the beginning. As a result, any observed differences in posttest outcomes might be caused by the treatment, but they could equally be the result of these pre-existing confounding variables.

Posttest Only Nonequivalent Groups Design

A pretest-posttest nonequivalent groups design enhances the posttest only approach by introducing an initial baseline measurement. This quasi-experimental design involves two groups: a treatment group that receives a pretest, the intervention, and a posttest; and a nonequivalent control group that takes both tests without receiving the intervention. Since participants are not randomly assigned, comparing the pretest-to-posttest changes between the groups allows researchers to evaluate if those who received the treatment demonstrated significantly greater improvement than those who did not, helping to account for general maturation or historical effects.

Pretest-Posttest Nonequivalent Groups Design

An interrupted time-series design with nonequivalent groups is a quasi-experimental approach that enhances the basic interrupted time-series design by incorporating a control group. This methodology involves taking multiple measurements of a dependent variable at regular intervals over an extended period both before and after an intervention is implemented across two or more groups that were not randomly assigned. By comparing the baseline and post-intervention trends of the treatment group to those of the nonequivalent control group, researchers can obtain stronger evidence regarding the effectiveness of the intervention.

Interrupted Time-Series Design with Nonequivalent Groups

What is the defining characteristic of a nonequivalent groups design?

Because a nonequivalent groups design does not use random assignment, researchers must expect that their comparison groups will likely have pre-existing differences that could threaten the internal validity of their research.

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