The What Works Clearinghouse (WWC) Standards Briefs explain the rules the WWC uses to evaluate the quality of studies for practitioners, researchers, and policymakers. This brief explains what baseline equivalence is and why it matters. As part of the WWC review process for certain types of studies, reviewers assess whether the intervention group (those that received the intervention of interest) and the comparison group (those that did not receive the intervention) were similar at the start of the study. If these groups are similar using a standardized mean difference called an effect size, the WWC determines that the study demonstrates baseline equivalence.