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The quasi-experimental design is frequently the most practical option for conducting outcome evaluations in the social services context. By using preexisting or self-selected groups, such as individuals already enrolled in a program, it avoids the additional steps required with random assignment to study conditions, as well as the potential ethical concerns involved in withholding or delaying treatment or substituting a less effective treatment for one group of study participants. The significant limitation of this design is that without randomization, the study groups may differ in important ways that account for some of the group differences in outcomes after the intervention. In other words, this design provides comparatively weaker evidence of program effects than one that uses randomization.

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