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.