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 What Works Clearinghouse (WWC) study reviews, certified reviewers look for a specific type of confounding factor: those that occur when a component of the study design or the circumstances under which the intervention was implemented are perfectly aligned with either the intervention or comparison group. That is, some factor is always present for members of one group and never present for members in the other group. Confounding factors can occur in any study design, even when group members are assigned to the intervention and comparison groups at random. The WWC is an initiative of the U.S. Department of Education's Institute of Education Sciences. The WWC evaluates research studies that look at the effectiveness of education programs, policies, and practices, which the WWC calls "interventions." WWC Standards Briefs explain the rules the WWC uses to assess the quality of studies. This brief discusses: (1) What is a confounding factor?; (2) Why do confounding factors matter?; (3) What types of confounding factors commonly occur in WWC-reviewed studies?; and (4) How does a confounding factor affect a study's WWC rating? [For more information about the review process, see "Standards Handbook. Version 4.0. What Works Clearinghouse™," ED577036 and "Procedures Handbook. Version 4.0. What Works Clearinghouse™," ED577035.]