The What Works Clearinghouse (WWC) evaluates research studies that look at the effectiveness of education programs, products, practices, and policies, which the WWC calls "interventions." Many studies of education interventions make claims about impacts on students' outcomes. Some studies have designs that enable readers to make causal inferences about the effects of an intervention but others have designs that do not permit these types of conclusions. To help policymakers, practitioners, and others make sense of study results, the WWC conducts a thorough assessment of the quality of evidence the study provided on the effectiveness of an intervention, called a WWC study review. This brief explains how individual reviewers assess studies, how ratings are confirmed and documented, and what happens if a study does not provide all the information a reviewer needs.