The What Works Clearinghouse (WWC) Standards Briefs explain the rules the WWC uses to evaluate the quality of studies for practitioners, researchers, and policymakers. An aspect of a study is considered a confounding factor if it is not possible to tell whether the difference in outcomes is due to the intervention, the confounding factor, or both. In WWC study reviews, certified reviewers look for confounding factors that are always present for members of one group and never present for members of the other group. This brief explains why confounding factors matter, what types of confounding factors commonly occur in WWC-reviewed studies, and why studies with certain types of confounding factors cannot meet WWC design standards.