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A matched-groups design serves as an alternative to simple random assignment. In this approach, researchers first measure the dependent variable or a relevant extraneous variable to assess the participants. They then rank-order the participants based on this measurement and randomly assign the most closely matched individuals to different experimental conditions. This technique guarantees that the groups are equivalent on that specific variable at the start of the study, preventing it from becoming a confounding variable.

Matched-Groups Design

To illustrate a matched-groups design, consider a study testing the effect of expressive writing on physical health. Researchers first measure baseline health variables to rank-order all participants from healthiest to least healthy. Then, taking the two healthiest individuals, they randomly assign one to the traumatic writing condition and the other to the neutral writing condition. They repeat this paired random assignment for the next two healthiest participants, continuing down the list. This process guarantees that the two groups have equivalent average health before the writing manipulation begins.

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